If you use Mac OS X Server 10.6 (Snow Leopard Server) as a mail server, by default it rejects mails without a subject line.

If you want to change this behavior, you just need to open the file /etc/postfix/main.cf and change the line

header_checks = pcre:/etc/postfix/custom_header_checks

to

# header_checks = pcre:/etc/postfix/custom_header_checks

Afterwards, type

sudo postfix reload

to activate the new setting.

You could also change the rule set in /etc/postfix/custom_header_checks to do the same thing, yet, as currently the only rule in this file is to reject incoming mails with an empty subject, we can easily ignore the whole file like described above.