Q We run Apple’s mail server in OS X Server, and our users are complaining that it rejects messages with an empty subject header. Is this a default, and how can we change it?

A Yes, this is one of the default settings in Apple’s Postfix mail server. To change it, open the text file at /etc/postfix/custom_header_checks and look for a line reading
/^Subject: *$/ REJECT Empty subject
Comment this out by putting a hash symbol # at the start of the line, save the file, and load it up with the command
sudo postfix reload

More recent versions of Postfix may store this file in a different folder, such as in Library/Server/Mail/Config/postfix, or even inside the Server app itself, but the fix remains the same wherever the file is now hidden.

Updated from the original, which was first published in MacUser volume 26 issue 16, 2010.