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

I just started working on a Mac OS X Server book for O’Reilly (in german).
For that reason I am going to be back for consultancy jobs in early December.

I will answer my emails before that, though…