https://issues.apache.org/SpamAssassin/show_bug.cgi?id=6678

--- Comment #7 from D. Stussy <[email protected]> 
2011-10-20 08:22:34 UTC ---
Note/aside:  Sometimes, I use MSOE to post to Usenet and set a reply-to to a
special mailbox which I have programmed my MTA to accept only reply messages. 
It determines whether a message is a reply by actually scanning the References
and In-Reply-To headers for a message ID issued by my host or NNTP server. 
When it fails to find one, it SMTP rejects the message.  Therefore, I am quite
certain that MSOE generates these headers properly (since that's what I used to
generate test messages for my MTA rulesets).  I do not require that the subject
start with "RE:" because a reply could change the topic and thus follow a
format of "<new_subject> - was RE: <old_subject>."

I have seen spammers try to send to my reserved mailbox after harvesting the
address from Usenet - and in every case, their message was rejected for not
having either of the ref/IRT headers.  I do look carefully at my logs when this
happens and I have yet to see a false positive spam.  So far, I have not had to
examine the local-part of the ref/IRT message IDs to verify that it was a
message I actually sent when spam was detected.  (That doesn't mean that I
don't examine the local-parts; all it means is that when spam was detected, the
domain-part didn't match, was absent, or the headers were missing.  I have yet
to see a spam that has a matching domain-part -- which could happen.)

Therefore, I suggest that starting a subject with "Re:" is some spammer's
attempt to bypass simple filters which may skip certain spam checks on the
grounds that it's a reply (especially for a C/R based system which expects a
reply in band).  "Re:" is merely a convention not present in any RFC, but the
Ref/IRT headers have been in the RFCs (5322 -> 2822 -> 822 ->733 ->724 [12 May
1977], Sections II.C.2.b and II.C.2.c) for 34 years.  "By definition," a reply
will have at least one if not both of these headers, even if it lacks "Re:" in
the subject.  Furthermore, any "true" reply which lacks both of these headers
probably is a fake or from a noncompliant mail user agent; either way, I don't
see the triggering of this rule as false.

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