Spam Filtering for Mail Exchangers:
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# 3.3. Forwarded Mail

You should take care not to reject mail as a result of spam filtering if it is forwarded from "friendly" sources, such as:

- Your backup MX hosts, if any. Supposedly, these have already filtered out most of the junk (see Multiple Incoming Mail Exchangers).

- 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.

- Other accounts belonging to the recipient. Again, rejections will generate collateral spam, and/or create problems for the host that forwards the mail.

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?

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 RCPT TO: 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 aggregate of each recipient's whitelist.

The implementation appendices cover this in more detail.

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