On Sun, 2014-10-05 at 12:51 +0100, Dennis Davis wrote:

On Sat, 4 Oct 2014, lee wrote:

...

Is the Subject: header required by some RFC? If it is, I'll also deny messages that don't have one.

Not as far as I'm aware. RFC5322, "Internet Message Format", contains:

The only required header fields are the origination date field and the originator address field(s). All other header fields are syntactically optional. More information is contained in the table following this definition.

There is a real difference between the lack of a Subject: header and a present but empty Subject: header.

I reject emails with no Subject: headers but accept empty Subject: headers because even I can occasionally forget to type-in a Subject.