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>3.3. Forwarded Mail</H1
><P
> You should take care not to reject mail as a result of spam
filtering if it is forwarded from <SPAN
CLASS="QUOTE"
>"friendly"</SPAN
>
sources, such as:
</P
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> Your backup MX hosts, if any. Supposedly, these have
already filtered out most of the junk (see <A
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>Multiple Incoming Mail Exchangers</A
>).
</P
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> Mailing lists, to which you or your users subscribe. You may
still filter such mail (it may not be as criticial if it
ends up in a black hole). However, if you reject the mail,
you may end up causing the list server to automatically
unsubscribe the recipient.
</P
></LI
><LI
><P
> Other accounts belonging to the recipient. Again,
rejections will generate collateral spam, and/or create
problems for the host that forwards the mail.
</P
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><P
> You may see a logistical issue with the last two of these
sources: They are specific to each recipient. How to you allow
each user to specify which hosts they want to whitelist, and
then use such individual whitelists in a system-wide SMTP-time
filtering setup? If the message is forwarded to several
recipients at your site (as may often be true in the case of
a mailing list), how do you decide whose whitelist to use?
</P
><P
> There is no magic bullet here. This is one of those situations
where we just have to do a bit of work. You can decide to
accept all mails, regardless of spam classification, so long as
it is sent from a host in the whitelist of any one of the
recipients. For instance, in response to each <B
CLASS="command"
>RCPT
TO:</B
> command, we can match the sending host against the
corresponding user's whitelist. If found, set a flag that will
prevent a subsequent rejection. Effectively, you are using an
<EM
>aggregate</EM
> of each recipient's whitelist.
</P
><P
> The implementation appendices cover this in more detail.
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