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I have set up a linux email server with Postfix and have correctly set up SPF and DKIM. When I send an email, the email is signed properly, and when I view the source from the receiving client (such as gmail), I can see that SPF and DKIM passed.

I also have dovecot installed so that I can use POP3 and IMAP. Currently, I use gmail as a POP3 client so that I can take advantage of their wonderful anti-spam filters. While the actual account on the server may have spam, when I check the email via gmail, the spam gets filtered properly.

The problem I'm having is with bounced emails. If I send an email to a non-existant address, the message is bounced, and then Postfix sends me a bounce email, saying something like "the remote mail system said the mailbox is unavailable".

This bounce email is not being signed with DKIM, which I can see when looking at the source. Therefore, my POP3 client (ie gmail) is marking the bounce email as spam.

What must I do in order to have my bounce email also go through DKIM processing?

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I'm afraid that according to the docs you're out of luck trying to make it work with Postfix:

Postfix currently does not apply content filters to mail that is forwarded or aliased internally, or to mail that is generated internally such as bounces or Postmaster notifications. This may be a problem when you want to apply a signing Milter to such mail.

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