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<h3><a name="SECTION0016210000000000000000">
6.2.10 SMTP Callback</a>
</h3>

<p>
Exim's SMTP callback feature is an even more powerful way to detect bogus
sender addresses than normal sender verification.  Unfortunately, lots of
servers send bounce messages with a bogus address in the header, and there are
plenty that send bounces with bogus envelope senders (even though they're
supposed to just use an empty envelope sender for bounces).

<p>
In order to ensure that Mailman can disable/remove bouncing addresses, you
generally want to receive bounces for Mailman lists, even if those bounces are
themselves not bounceable.  Thus, you might want to disable SMTP callback on
bounce messages.

<p>
With Exim 4, you can accomplish this using something like the following in
your <code>RCPT TO</code> ACL:

<p>
<div class="verbatim"><pre>
  # Accept bounces to lists even if callbacks or other checks would fail
  warn     message      = X-WhitelistedRCPT-nohdrfromcallback: Yes
           condition    = \
           ${if and {{match{$local_part}{(.*)-bounces\+.*}} \
                     {exists {MAILMAN_HOME/lists/$1/config.pck}}} \
                {yes}{no}}

  accept   condition    = \
           ${if and {{match{$local_part}{(.*)-bounces\+.*}} \
                     {exists {MAILMAN_HOME/lists/$1/config.pck}}} \
                {yes}{no}}

  # Now, check sender address with SMTP callback.
  deny   !verify = sender/callout=90s
</pre></div>

<p>
If you also do SMTP callbacks on header addresses, you'll want something like
this in your <code>DATA</code> ACL:

<p>
<div class="verbatim"><pre>
  deny   !condition = $header_X-WhitelistedRCPT-nohdrfromcallback:
         !verify = header_sender/callout=90s
</pre></div>
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<span class="release-info">Release 2.1, documentation updated on March 7, 2015.</span>
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