I noticed the MISSING_SUBJECT rule matched for every mail recently even though the mails contained a subject header. Finally I found this in the spamd.log:

warn: Possible unintended interpolation of @yahoo in string at /etc/spamassassin/local.cf, rule MY_SPAMMY_YAHOO, line 1.
warn: rules: failed to compile Mail::SpamAssassin::Plugin::Check::_head_tests_0_4, skipping:
warn:  (Global symbol "@yahoo" requires explicit package name at /etc/spamassassin/local.cf, rule MY_SPAMMY_YAHOO, line 1.)

The reason was a rule like this:

header          MY_SPAMMY_YAHOO        To =~ /myspammyaddress@yahoo.de/
score           MY_SPAMMY_YAHOO        3
describe        MY_SPAMMY_YAHOO        Mail to myspammyaddress@yahoo.de is mostly spam

Escaping the @ in the rule fixed the Problem:

header          MY_SPAMMY_YAHOO        To =~ /myspammyaddress\@yahoo.de/

Hope this helps someone to spot the error faster.

The problem is that spamassasin does not autolearn ham if the ham messages match the MISSING_SUBJECT rule. So an error like this basically not only breaks lots of tests but also autolearning of ham. So lesson learned: better always look at the spamd.log after creating a new rule, even if you think you know what you are doing 😉