Blocking empty subject line emails from being sent or received?

How can I configure Postfix, Dovecot, and/or SpamAssasin to reject all incoming and outgoing emails with empty subject line emails?

I am running on a Debian 8.6.

If you want to REJECT the mail, which would be a good idea as it also informs the sender that you don't want to receive mail with empty subject, you should use Postfix header_checks.

Add header_checks = pcre:/etc/postfix/header_checks to your main.conf and then add there this regular expression for empty Subject:

/^subject: *$/   REJECT  Please add subject to your mail.

In order to use this Perl Compatible Regular Expression you must have Postfix PCRE map support (from package postfix-pcre) installed.

Only limitation with header_checks is that it cannot recognize if the Subject field is totally missing. This problem you could address by scoring it higher on SpamAssassin. For global configuration /etc/spamassassin/local.cf and for user configuration ~/.spamassassin/user_prefs add:

score MISSING_SUBJECT          30

If you need to REJECT mails without Subject header, not just flag and/or quietly remove the mail, you can make SpamAssasin reject mail based on score by using it as a proxy-filter for Postfix.