It would help if you told what version of OS X server you are using.
Most will have something like that set in the postfix rules by default. Look in this file:
/etc/postfix/custom_header_checks
(this is where the version matters... ML and later are in /Library/Server/Mail/Config, unless the admin has rebuilt postfix and put it back in the default locations... like I do)
Search for a line with "empty subject header" and if it exists, comment it out (add "#" to the beginning). Then you can tell postfix to reload the configuration (postfix reload).
It's all good as long as it works. I have mine in /etc/postfix too on 10.7 and 10.10, I think because they are servers they are there. If you're not using any of the postfix rules (I use them extensively) then it might be barren...
Try "postconf -h header_checks" and make sure that is the file you've edited. It looks like the error from postfix, but if you can do a test with a mail client and send an empty subject mail, you can verify that postfix is causing the rejection.
bash-3.2# postconf -h header_checks
bash-3.2#
/^subject: *$/ REJECT empty subject header
bash-3.2# postconf -h header_checks
bash-3.2# pico /etc/postfix/custom_header_checks
bash-3.2# postfix reload
postfix/postfix-script: refreshing the Postfix mail system
bash-3.2# postconf -h header_checks
bash-3.2#
That command gives no result. Ie:
If I understand it correctly that *should* give info about rejecting empty headers, right!?