MailScanner causes SpamAssassin rules to firing inconsistently

[Wolfgang Baudler]

We run MailScanner 4.85.2 with SpamAssassin 3.4.1 on RHEL6. Bayes and
Autolearn are disabled.

I tried to add a custom rule to SpamAssassin local.cf and found that it
fires inconsistently. A simple example rule is

body    TEST_RULE_AA            /SOMETEXT/
score   TEST_RULE_AA            0.5

If I send a test email locally with the string "SOMETEXT" the rule
triggers all the time. If I run SpamAssassin from the command line, the
rule triggers as well

$ spamassassin <test.msg

If I send a test message from an external provider like yahoo however the
rule does not always fire (some other rules like FREEMAIL_FROM do fire).

The peculiar thing is that they do fire some times if I resend the message
often enough. I can not figure out what is causing these inconsistencies.

This is a very basic body rule, which should fire unconditionally all the
time it finds the string. It only has one score, so SpamAssassin score set
changes shouldn't play into this either.

What am I missing? Why are the rules not applied consistently for every
email when SpamAssassin is run through MailScanner (and ONLY when it is
run through MailScanner, sapmassassin detects it 100% of the time when run
manually)?

Any suggestions appreciated.

Wolfgang