How to specify a partial domain in blacklist with SpamAssassin

There are people who claim to have gotten what you're describing to work by installing the Intelligent Message Filter and modifying the "MSExchange.UceContentFilter.xml" configuration file. You can create "CustomWeightEntry" records in that file that match text that SpamAssassin puts into the subject like and use that to assign a custom SCL.

Have a look here:

• http://technet.microsoft.com/en-us/library/aa996624(EXCHG.65).aspx
• http://technet.microsoft.com/en-us/magazine/2006.10.weightlists.aspx?pr=blog

Personally I've never tried it. It looks pretty straightforward, though.

You can delete emails with a particular tag in header.

See Postfix Header Checks Man page for details.

Example:

In "/etc/postfix/main.cf", add

header_checks = regexp:/etc/postfix/header_checks

In "/etc/postfix/header_checks", have

/^(Subject: \[SPAM\])(.+)$/ DISCARD

The above will discard all emails whose Subject starts with "[SPAM]".

EDIT: I only wish to delete the messages that are both spam AND being forwarded to other servers. I don't want to delete all mails marked as spam. Is there a way to distinguish between the two?

This could be done by running two instances within Postfix. The first instance will forward the email to second instance (instead of to Remote email server). The same can be achieved by specifying a local transport.

The second instance will have the above header_checks. This way only emails to remote server will have their SPAM deleted.

Some references:

1. Specifying Transport
2. You can run multiple instances of Postfix by specifying additional entries in master.cf file. Details here