Is Mac OS X Server 10.6 (Snow Leopard) rejecting your e-mail messages which lack a subject line? Mine was, and so does the default setting on Snow Leopard Server.
Here’s a quick fix:
1) First, stop the mail service.
2 Now, you’ll want to edit the header_checks file that’s utilized by postfix.
sudo nano /etc/postfix/custom_header_checks
3) Next, you’ll see a line which says:
/^subject: *$/ REJECT empty subject header
Comment out this line by adding a pound sign (#) at the beginning of the line.
4) Once you comment out this line, it should appear as:
#/^subject: *$/ REJECT empty subject header
5) If you’re using nano or pico to edit the file, click Control+O to write the file. (Save and Exit).
6) Now, restart the Mail service and the server will no longer reject email with no subject line.
Let me know if this solution works for you! Enjoy!