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).