One of our techs ran across this problem and someone posted a fix in Apple’s Support Forums.
In case the link changes/goes down here’s the salient portion of the suggested fix:
Turns out that SL rejects empty subject lines by default. In order to make it play nice I stopped the mail service headed over to:
/^subject: *$/ REJECT empty subject header in /etc/
#/^subject: *$/ REJECT empty subject header in /etc/
After restarting mail all worked perfectly.