<reference>
    <title>@PACKAGE@ Sendmail milter - Version @VERSION@</title>
    <partintro>
        <title>Packages</title>
        <para>The various source and binary packages are available at <ulink
        url="http://www.five-ten-sg.com/@PACKAGE@/packages">http://www.five-ten-sg.com/@PACKAGE@/packages</ulink>
        The most recent documentation is available at <ulink
        url="http://www.five-ten-sg.com/@PACKAGE@/">http://www.five-ten-sg.com/@PACKAGE@/</ulink>
        </para>

    </partintro>

    <refentry id="@PACKAGE@.1">
        <refentryinfo>
            <date>2005-12-18</date>
        </refentryinfo>

        <refmeta>
            <refentrytitle>@PACKAGE@</refentrytitle>
            <manvolnum>1</manvolnum>
            <refmiscinfo>@PACKAGE@ @VERSION@</refmiscinfo>
        </refmeta>

        <refnamediv id='name.1'>
            <refname>@PACKAGE@</refname>
            <refpurpose>a sendmail milter with per-user dnsbl filtering</refpurpose>
        </refnamediv>

        <refsynopsisdiv id='synopsis.1'>
            <title>Synopsis</title>
            <cmdsynopsis>
                <command>@PACKAGE@</command>
                <arg><option>-c</option></arg>
                <arg><option>-s</option></arg>
                <arg><option>-d <replaceable class="parameter">n</replaceable></option></arg>
                <arg><option>-e <replaceable class="parameter">from|to</replaceable></option></arg>
                <arg><option>-r <replaceable class="parameter">local-domain-socket</replaceable></option></arg>
                <arg><option>-p <replaceable class="parameter">sendmail-socket</replaceable></option></arg>
                <arg><option>-t <replaceable class="parameter">timeout</replaceable></option></arg>
            </cmdsynopsis>
        </refsynopsisdiv>

        <refsect1 id='options.1'>
            <title>Options</title>
            <variablelist>
                <varlistentry>
                    <term>-c</term>
                    <listitem>
                        <para>
                            Load the configuration file, print a cannonical form
                            of the configuration on stdout, and exit.
                       </para>
                   </listitem>
                </varlistentry>
                <varlistentry>
                    <term>-s</term>
                    <listitem>
                        <para>
                            Stress test the configuration loading code by repeating
                            the load/free cycle in an infinite loop.
                       </para>
                   </listitem>
                </varlistentry>
                <varlistentry>
                    <term>-d <replaceable class="parameter">n</replaceable></term>
                    <listitem>
                        <para>
                            Set the debug level to <replaceable class="parameter">n</replaceable>.
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>-e <replaceable class="parameter">from|to</replaceable></term>
                    <listitem>
                        <para>
                            Print the results of looking up the from and to addresses in the
                            current configuration. The | character is used to separate the from and to
                            addresses in the argument to the -e switch.
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>-r <replaceable class="parameter">local-domain-socket</replaceable></term>
                    <listitem>
                        <para>
                            Set the local socket used for the connection to our own dns resolver processes.
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>-p <replaceable class="parameter">sendmail-socket</replaceable></term>
                    <listitem>
                        <para>
                            Set the socket used for the milter connection to sendmail. This is either
                            "inet:port@ip-address" or "local:local-domain-socket-file-name".
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>-t <replaceable class="parameter">timeout</replaceable></term>
                    <listitem>
                        <para>
                            Set the timeout in seconds used for communication with sendmail.
                        </para>
                    </listitem>
                </varlistentry>
            </variablelist>
        </refsect1>

        <refsect1>
            <title>Usage</title>
            <para><command>@PACKAGE@</command> -c</para>
            <para><command>@PACKAGE@</command> -s</para>
            <para><command>@PACKAGE@</command> -d 2</para>
            <para><command>@PACKAGE@</command> -e'someone@aol.com|localname@mydomain.tld'</para>
            <para><command>@PACKAGE@</command> -d 10 -r /var/run/dnsbl/dnsbl.resolver.sock -p local:/var/run/dnsbl/dnsbl.sock</para>
        </refsect1>

        <refsect1 id='introduction.1'>
            <title>Introduction</title>
            <para>
                Consider the case of a mail server that is acting as secondary MX for a
                collection of clients, each of which has a collection of mail domains.
                Each client may use their own collection of DNSBLs on their primary mail
                server.  We present here a mechanism whereby the backup mail server can
                use the correct set of DNSBLs for each recipient for each message.  As a
                side-effect, it gives us the ability to customize the set of DNSBLs on a
                per-recipient basis, so that fred@example.com could use SPEWS and the
                SBL, where all other users @example.com use only the SBL.
            </para>
            <para>
                This milter can also verify the envelope from/recipient pairs with the
                primary MX server.  This allows the backup mail servers to properly
                reject mail sent to invalid addresses.  Otherwise, the backup mail
                servers will accept that mail, and then generate a bounce message when
                the message is forwarded to the primary server (and rejected there with
                no such user).
            </para>
            <para>
                This milter will also decode (uuencode, base64, mime, html entity, url
                encodings) and scan for HTTP and HTTPS URLs and bare hostnames in the
                body of the mail.  If any of those host names have A or NS records on
                the SBL (or a single configurable DNSBL), the mail will be rejected
                unless previously whitelisted.  This milter also counts the number of
                invalid HTML tags, and can reject mail if that count exceeds your
                specified limit.
            </para>
            <para>
                The DNSBL milter reads a text configuration file (dnsbl.conf) on
                startup, and whenever the config file (or any of the referenced include
                files) is changed.  The entire configuration file is case insensitive.
                If the configuration cannot be loaded due to a syntax error, the milter
                will log the error and quit.  If the configuration cannot be reloaded
                after being modified, the milter will log the error and send an email to
                root from dnsbl@$hostname.  You probably want to added dnsbl@$hostname
                to your /etc/mail/virtusertable since otherwise sendmail will reject
                that message.
            </para>
        </refsect1>

        <refsect1 id='todo.1'>
            <title>DCC Issues</title>
            <para>
                If you are also using the <ulink
                url="http://www.rhyolite.com/anti-spam/dcc/">DCC</ulink> milter, there
                are a few considerations.  You may need to whitelist senders from the
                DCC bulk detector, or from the DNS based lists.  Those are two very
                different reasons for whitelisting.  The former is done thru the DCC
                whiteclnt config file, the later is done thru the DNSBL milter config
                file.
            </para>
            <para>
                You may want to blacklist some specific senders or sending domains.
                This could be done thru either the DCC (on a global basis, or for a
                specific single recipient).  We prefer to do such blacklisting via the
                DNSBL milter config, since it can be done for a collection of recipient
                mail domains.  The DCC approach has the feature that you can capture the
                entire message in the DCC log files.  The DNSBL milter approach has the
                feature that the mail is rejected earlier (at RCPT TO time), and the
                sending machine just gets a generic "550 5.7.1 no such user" message.
            </para>
            <para>
                The DCC whiteclnt file can be included in the DNSBL milter config by the
                dcc_to and dcc_from statements.  This will import the (env_to, env_from,
                and substitute mail_host) entries from the DCC config into the DNSBL
                config.  This allows using the DCC config as the single point for
                white/blacklisting.
            </para>
            <para>
                Consider the case where you have multiple clients, each with their own
                mail servers, and each running their own DCC milters.  Each client is
                using the DCC facilities for envelope from/to white/blacklisting.
                Presumably you can use rsync or scp to fetch copies of your clients DCC
                whiteclnt files on a regular basis.  Your mail server, acting as a
                backup MX for your clients, can use the DNSBL milter, and include those
                client DCC config files.  The envelope from/to white/blacklisting will
                be appropriately tagged and used only for the domains controlled by each
                of those clients.
            </para>
        </refsect1>

        <refsect1 id='todo.1'>
            <title>Definitions</title>
            <para>
                CONTEXT - a collection of parameters that defines the filtering context
                to be used for a collection of envelope recipient addresses.  The
                context includes such things as the list of DNSBLs to be used, and the
                various content filtering parameters.
            </para>
            <para>
                DNSBL - a named DNS based blocking list is defined by a dns suffix (e.g.
                sbl-xbl.spamhaus.org) and a message string that is used to generate the
                "550 5.7.1" smtp error return code.  The names of these DNSBLs will be
                used to define the DNSBL-LISTs.
            </para>
            <para>
                DNSBL-LIST - a named list of DNSBLs that will be used for specific
                recipients or recipient domains.
            </para>
        </refsect1>

        <refsect1 id='todo.1'>
            <title>Filtering Procedure</title>
            <para>
                If the client has authenticated with sendmail, the mail is accepted, the
                filtering contexts are not used, the dns lists are not checked, and the
                body content is not scanned.  Otherwise, we follow these steps for each
                recipient.
            </para>
            <orderedlist>
                <listitem>
                    The envelope to email address is used to find an initial filtering
                    context.  We first look for a context that specified the full email
                    address in the env_to statement.  If that is not found, we look for a
                    context that specified the entire domain name of the envelope recipient
                    in the env_to statement.  If that is not found, we look for a context
                    that specified the user@ part of the envelope recipient in the env_to
                    statement.  If that is not found, we use the first top level context
                    defined in the config file.
                </listitem>
                <listitem>
                    The initial filtering context may redirect to a child context based on
                    the values in the initial context's env_from statement.  We look for [1)
                    the full envelope from email address, 2) the domain name part of the
                    envelope from address, 3) the user@ part of the envelope from address]
                    in that context's env_from statement, with values that point to a child
                    context.  If such an entry is found, we switch to that child filtering
                    context.
                </listitem>
                <listitem>
                    We lookup [1) the full envelope from email address, 2) the domain name
                    part of the envelope from address, 3) the user@ part of the envelope
                    from address] in the filtering context env_from statement.  That results
                    in one of (white, black, unknown, inherit).
                </listitem>
                <listitem>
                    If the answer is black, mail to this recipient is rejected with "no such
                    user", and the dns lists are not checked.
                </listitem>
                <listitem>
                    If the answer is white, mail to this recipient is accepted and the dns
                    lists are not checked.
                </listitem>
                <listitem>
                    If the answer is unknown, we don't reject yet, but the dns lists will be
                    checked, and the content may be scanned.
                <listitem>
                    If the answer is inherit, we repeat the envelope from search in the
                    parent context.
                </listitem>
                <listitem>
                    The dns lists specified in the filtering context are checked and the
                    mail is rejected if any list has an A record for the standard dns based
                    lookup scheme (reversed octets of the client followed by the dns
                    suffix).
                </listitem>
                <listitem>
                    If the mail has not been accepted or rejected yet, we look for a
                    verification context, which is the closest ancestor of the filtering
                    context that both specifies a verification host, and which covers the
                    envelope to address.  If we find such a verification context, and the
                    verification host is not our own hostname, we open an smtp conversation
                    with that verification host.  The current envelope from and recipient to
                    values are passed to that verification host.  If we receive a 5xy
                    response those commands, we reject the current recipient with "no such
                    user".
                </listitem>
                <listitem>
                    If the mail has not been accepted or rejected yet, and the filtering
                    context enables content filtering, and this is the first such recipient
                    in this smtp transaction, we set the content filtering parameters from
                    this context, and enable content filtering for the body of this message.
                </listitem>
            </orderedlist>
            <para>
                If content filtering is enabled for this body, the mail text is decoded
                (uuencode, base64, mime, html entity, url encodings), scanned for HTTP
                and HTTPS URLs, and the first &lt;configurable&gt; host names are
                checked for their presence on the single &lt;configurable&gt; DNSBL.
                The only known list that is suitable for this purpose is the SBL.  If
                any of those host names are on that DNSBL (or have nameservers that are
                on that list), and it is not on the &lt;configurable&gt; ignore list,
                the mail is rejected.  We also scan for excessive bad html tags, and if
                a &lt;configurable&gt; limit is exceeded, the mail is rejected.
            </para>
        </refsect1>

        <refsect1>
            <title>Sendmail access vs. DNSBL</title>
            <para>
                With the standard sendmail.mc dnsbl FEATURE, the dnsbl checks may be
                suppressed by entries in the /etc/mail/access database.  For example,
                suppose you control a /18 of address space, and have allocated some /24s
                to some clients.  You have access entries like
                <screen>
                    192.168.4   OK
                    192.168.17  OK
                </screen>
            </para>
            <para>
                to allow those clients to smarthost thru your mail server.  Now if one
                of those clients happens get infected with a virus that turns a machine
                into an open proxy, and their 192.168.4.45 lands on the SBL-XBL, you
                will still wind up allowing that infected machine to smarthost thru your
                mail servers.
            </para>
            <para>
                With this DNSBL milter, the sendmail access database cannot override the
                dnsbl checks, so that machine won't be able to send mail to or thru your
                smarthost mail server (unless the virus/proxy can use smtp-auth).
            </para>
            <para>
                Using the standard sendmail features, you would add access entries to
                allow hosts on your local network to relay thru your mail server.  Those
                OK entries in the sendmail access database will override all the dnsbl
                checks.  With this DNSBL milter, you will need to have the local users
                authenticate with smtp-auth to get the same effect.  You might find
                <ulink
                url="http://www.ists.dartmouth.edu/classroom/sendmail-ssl-how-to.php">
                these directions</ulink> helpful for setting up smtp-auth if you are on
                RH Linux.
            </para>
        </refsect1>

        <refsect1>
            <title>Installation and configuration</title>
            <para>
                This is a standard GNU autoconf/automake installation, so the normal
                <screen>
                    ./configure
                    make
                    su
                    make install
                </screen>
                works. "make chkconfig" will setup the init.d runlevel scripts.
            </para>
            <para>
                Note that this has ONLY been tested on Linux, specifically RedHat Linux.
                In particular, this milter makes no attempt to understand IPv6.  Your
                mileage will vary.  You will need at a minimum a C++ compiler with a
                minimally thread safe STL implementation.  The distribution includes a
                test.cpp program.  If it fails this milter won't work.  If it passes,
                this milter might work.
            </para>
            <para>
                Modify your sendmail.mc by removing all the "FEATURE(dnsbl" lines, add
                the following line in your sendmail.mc and rebuild the .cf file
            </para>
            <para>
                <screen>
                    INPUT_MAIL_FILTER(`dnsbl', `S=local:/var/run/dnsbl/dnsbl.sock, F=T, T=C:30s;S:5m;R:5m;E:5m')
                </screen>
            </para>
            <para>
                Modify the default <citerefentry>
                <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum>
                </citerefentry> configuration.
            </para>


        <refsect1 id='todo.1'>
            <title>Performance Issues</title>
            <para>
                Consider a high volume high performance machine running sendmail.  Each
                sendmail process can do its own dns resolution.  Typically, such dns
                resolver libraries are not thread safe, and so must be protected by some
                sort of mutex in a threaded environment.  When we add a milter to
                sendmail, we now have a collection of sendmail processes, and a
                collection of milter threads.
            </para>
            <para>
                We will be doing a lot of dns lookups per mail message, and at least
                some of those will take many tens of seconds.  If all this dns work is
                serialized inside the milter, we have an upper limit of about 25K mail
                messages per day.  That is clearly not sufficient for many sites.
            </para>
            <para>
                Since we want to do parallel dns resolution across those milter threads,
                we add another collection of dns resolver processes.  Each sendmail
                process is talking to a milter thread over a socket, and each milter
                thread is talking to a dns resolver process over another socket.
            </para>
            <para>
                Suppose we are processing 20 messages per second, and each message
                requires 20 seconds of dns work.  Then we will have 400 sendmail
                processes, 400 milter threads, and 400 dns resolver processes.  Of
                course that steady state is very unlikely to happen.
            </para>
        </refsect1>


        <refsect1 id='todo.1'>
            <title>Rejected Ideas</title>
            <para>
                The following ideas have been considered and rejected.
            </para>
            <para>
                Add max_recipients for each mail domain to the configuration.
                Recipients in excess of that limit will be rejected, and all the
                recipients in that domain will be removed if there are some other
                whitelisted recipients.  Current spammers *very* rarely send more than
                ten recipients in a single smtp transaction, so this won't stop any
                significant amount of spam.
            </para>
            <para>
                Add poison addresses to the configuration.  If any recipient is
                poison, all recipients are rejected even if they would be whitelisted,
                and the data is rejected if sent.  I have a collection of spam trap
                addresses that would be suitable for such use.  Based on my log files,
                any mail to those spam trap addresses is rejected based on either dnsbl
                lookups or the DCC.  So this won't result in blocking any additional
                spam.
            </para>
            <para>
                Add an option to only allow one recipient if the return path is
                empty.  Based on my log files, there is no mail that violates this
                check.
            </para>
            <para>
                Reject the mail if the envelope from domain name contains any MX
                records pointing to 127.0.0.0/8.  I don't see any significant amount of
                spam sent with such domain names.
            </para>
        </refsect1>

        <refsect1 id='todo.1'>
            <title>TODO</title>
            <para>
                The following ideas are under consideration.
            </para>
            <para>
                Add a per-context option to reject mail if the number of digits in
                the reverse dns client name exceeds some threshold.
            </para>
        </refsect1>

        <refsect1>
            <title>Configuration</title>
            <para>
                The configuration file is documented in <citerefentry>
                <refentrytitle>@PACKAGE@.conf</refentrytitle> <manvolnum>5</manvolnum>
                </citerefentry>.  Any change to the config file, or any file included
                from that config file, will cause it to be reloaded within three
                minutes.
            </para>
        </refsect1>

        <refsect1>
            <title>Copyright</title>
            <para>
                Copyright (C) 2005 by 510 Software Group &lt;carl@five-ten-sg.com&gt;
            </para>
            <para>
                This program is free software; you can redistribute it and/or modify it
                under the terms of the GNU General Public License as published by the
                Free Software Foundation; either version 2, or (at your option) any
                later version.
            </para>
            <para>
                You should have received a copy of the GNU General Public License along
                with this program; see the file COPYING.  If not, please write to the
                Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
            </para>
        </refsect1>

        <refsect1>
            <para>
                $Id$
            </para>
        </refsect1>
    </refentry>


    <refentry id="@PACKAGE@.conf.5">
        <refentryinfo>
            <date>2005-12-18</date>
        </refentryinfo>

        <refmeta>
            <refentrytitle>@PACKAGE@.conf</refentrytitle>
            <manvolnum>5</manvolnum>
            <refmiscinfo>@PACKAGE@ @VERSION@</refmiscinfo>
        </refmeta>

        <refnamediv id='name.5'>
            <refname>@PACKAGE@.conf</refname>
            <refpurpose>configuration file for @PACKAGE@</refpurpose>
        </refnamediv>

        <refsynopsisdiv id='synopsis.5'>
            <title>Synopsis</title>
            <cmdsynopsis>
                <command>@PACKAGE@.conf</command>
            </cmdsynopsis>
        </refsynopsisdiv>

        <refsect1 id='description.5'>
            <title>Description</title>
            <para>The <command>@PACKAGE@.conf</command> configuration file is
            specified by this partial bnf description.</para>

            <literallayout class="monospaced"><![CDATA[
CONFIG     = {CONTEXT ";"}+
CONTEXT    = "context" NAME "{" {STATEMENT}+ "}"
STATEMENT  = (DNSBL | DNSBLLIST | CONTENT | ENV-TO | VERIFY | CONTEXT | ENV-FROM) ";"

DNSBL      = "dnsbl" NAME DNSPREFIX ERROR-MSG

DNSBLLIST  = "dnsbl_list" {NAME}+

CONTENT    = "content" ("on" | "off") "{" {CONTENT-ST}+ "}"
CONTENT-ST = (FILTER | IGNORE | TLD | HTML-TAGS | HTML-LIMIT | HOST-LIMIT) ";"
FILTER     = "filter" DNSPREFIX ERROR-MSG
IGNORE     = "ignore"     "{" {HOSTNAME [";"]}+ "}"
TLD        = "tld"        "{" {TLD      [";"]}+ "}"
HTML-TAGS  = "html_tags"  "{" {HTMLTAG  [";"]}+ "}"
ERROR-MSG  = string containing exactly two %s replacement tokens for the client ip address

HTML-LIMIT = "html_limit" ("on" INTEGER ERROR-MSG | "off")

HOST-LIMIT = "host_limit" ("on" INTEGER ERROR-MSG | "off" | "soft" INTEGER)

ENV-TO     = "env_to"     "{" {(TO-ADDR | DCC-TO)}+ "}"
TO-ADDR    = ADDRESS [";"]
DCC-TO     = "dcc_to" ("ok" | "many") "{" DCCINCLUDEFILE "}" ";"

VERIFY     = "verify" HOSTNAME ";"

ENV_FROM   = "env_from" [DEFAULT] "{" {(FROM-ADDR | DCC-FROM)}+ "}"
FROM-ADDR  = ADDRESS VALUE [";"]
DCC-FROM   = "dcc_from" "{" DCCINCLUDEFILE "}" ";"
DEFAULT    = ("white" | "black" | "unknown" | "inherit" | "")
ADDRESS    = (USER@ | DOMAIN | USER@DOMAIN)
VALUE      = ("white" | "black" | "unknown" | CHILD-CONTEXT-NAME)]]></literallayout>
        </refsect1>

        <refsect1 id='sample.5'>
            <title>Sample</title>
            <literallayout class="monospaced"><![CDATA[
context sample {
    dnsbl   local   blackholes.five-ten-sg.com  "Mail from %s rejected - local; see http://www.five-ten-sg.com/blackhole.php?%s";
    dnsbl   sbl     sbl-xbl.spamhaus.org        "Mail from %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s";
    dnsbl   xbl     xbl.spamhaus.org            "Mail from %s rejected - xbl; see http://www.spamhaus.org/query/bl?ip=%s";
    dnsbl   dul     dul.dnsbl.sorbs.net         "Mail from %s rejected - dul; see http://www.sorbs.net/lookup.shtml?%s";
    dnsbl_list  local sbl dul;

    content on {
        filter    sbl-xbl.spamhaus.org        "Mail containing %s rejected - sbl; see http://www.spamhaus.org/query/bl?ip=%s";
        ignore    { include "hosts-ignore.conf"; };
        tld       { include "tld.conf"; };
        html_tags { include "html-tags.conf"; };
        html_limit on 20 "Mail containing excessive bad html tags rejected";
        html_limit off;
        host_limit on 20 "Mail containing excessive host names rejected";
        host_limit soft 20;
    };

    env_to {
        # child contexts are not allowed to specify recipient addresses outside these domains
        # leave this outer global context env_to empty to allow arbitrary recipients in child contexts
        mydomain.com;
        customer1.com;
        customer1a.com;
        customer1b.com;
        customer2.com;
        customer2a.com;
        customer2b.com;
    };

    context whitelist {
        content off {};
        env_to {
            # dcc_to ok { include "/var/dcc/whitecommon"; };    # copy the dcc OK values (env_to) into this context
        };
        env_from white {};      # white forces all unmatched from addresses (everyone in this case) to be whitelisted
                                # so all mail TO these env_to addresses is accepted
    };

    context abuse {
        dnsbl_list xbl;
        content off {};
        env_to {
            abuse@;             # no content filtering on abuse reports
            postmaster@;        # ""
        };
        env_from unknown {};    # ignore all parent white/black listing
    };

    context minimal {
        dnsbl_list sbl dul;
        content on {};
        env_to {
            sales@mydomain.com;
        };
    };

    context blacklist {
        env_to {
            dcc_to many { include "/var/dcc/whitecommon"; };    # copy the dcc MANY values (env_to) into this context
            old-employee@mydomain.com;
        };
        env_from black {};      # black forces all unmatched from addresses (everyone in this case) to be blacklisted
                                # so all mail TO these env_to addresses is rejected
    };

    context vp {    # special context for the vp
        env_to {
            vp@mydomain.com;
        };
        env_from inherit {
            nai.com                 black;      # the vp does not like nai
            yahoo.com               unknown;    # override parent context blacklisting
            mother@spammyisp.com    white;      # suppress dnsbl checking
        };
    };

    context customer1 {
        dnsbl_list sbl dul;
        env_to {
            customer1.com;
            customer1a.com;
            customer1b.com;
        };

        verify mail.customer1.com;

        context customer1a {
            env_to {
                customer1a.com;
            }
            env_from black {                        # blacklist everything
                first@acceptable.com    unknown;    # except these specific envelope senders
                second@another.com      unknown;
                yahoo.com               inherit;    # delegate to the parent
            };
        };

        env_from {  # default value of the default is inherit
            yahoo.com           black;      # no mail from yahoo
            first@yahoo.com     unknown;    # except this one
        };
    };

    context customer2 {
        dnsbl_list sbl;
        env_to {
            customer2.com;
            customer2a.com;
            customer2b.com;
        };
    };

    env_from unknown {
        dcc_from { include "/var/dcc/whitecommon"; };   # copy the dcc OK/MANY values (env_from, substitute mail_host) into this context
        abuse@              abuse;      # replies to abuse reports use the abuse context
        yahoo.com           black;      # don't take mail from yahoo
        spammer@example.com black;
    };
};]]></literallayout>
        </refsect1>

        <refsect1>
            <para>
                $Id$
            </para>
        </refsect1>

    </refentry>
</reference>