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?