Here is the issue it seems. 2 headers are causing issue. The missing return-path is on FUDforum AFAIK. The missing "from" in one of the received headers is the mail server. So I am checking into the latter.
Using this online message syntax checker pasting in a rejected email that was generated from FUDforum...
MIME Lint v1.1.0 2012-09-08 is a strict syntax validator for Internet
messages including MIME, RFC 5322, DSN (RFC 3464) and MDN (RFC 3798) elements
which has be run on the attached message. The result follows:
-----------
WARNING: line too long in header 'Received:' at line 1
ERROR: Missing mandatory item 'from' in header 'Received' at lines 3-4
UNKNOWN: unknown header 'X-Spam-Checker-Version' at line 5
UNKNOWN: unknown header 'X-Spam-Level' at line 6
UNKNOWN: unknown header 'X-Spam-Status' at lines 7-8
WARNING: line too long in header 'Received:' at line 11
UNKNOWN: unknown header 'X-Mailer' at line 16
WARNING: duplicate header 'From' at line 17
WARNING: use SMTP MAIL FROM/return-path instead of non-standard errors-to in
header 'Errors-To' at line 18
UNKNOWN: unknown header 'X-Mailer' at line 19
ERROR: missing mandatory header 'return-path' lines 1-23
OK: found part text/plain line 25
WARNING: Content-Transfer-Encoding mislabelled as 8-bit when 7-bit suffices,
line 26
-----------
Output lines begin with 'OK:' for informational messages, 'UNKNOWN:' for
unregistered/unfamiliar extensions which may be incorrect, 'WARNING:' for
poor usage which is either likely to cause problems or fails the 'generate
conservative protocol' principle, and 'ERROR:' for standards violations. If
your result contains only 'OK:' results, your message passed MIME Lint v1.1.0
2012-09-08 validation.
There is no guarantee that this validator is free of bugs itself, so feel
free to contact <chris(dot)newman(at)oracle(dot)com> if you think you found an error in
the validator or have a good idea to enhance the validator.