Postfix doesn't sign messages with DKIM
There is a problem with postfix not signing messages with DKIM. (Original post: https://www.howtoforge.com/community/threads/mails-not-signed-with-dkim-amavis-not-setting-originating.72938/)
The problem is that, there is a parameter missing in the postfix/main.cf. Namely the smtpd_sender_restriction is missing the permit_sasl_authenticated value. This causes the tag_as_foreign filter to always overwrite the tag_as_originating filter and therefore handle all mail as foreign and thus not signing any mail.