Q: prevent Emacs from sending mail with no subject

[Eric Pement]

I would like to know how to prevent Emacs from sending email with no
subject (or with a subject line that contains only whitespace). I've
accidentally sent a couple of messages this way, and I think Emacs
converted the subject line to "(unknown)" for me, but I'm not sure. At
any rate, I want Emacs to not send the message if the subject line is
empty, but to prompt me to enter the missing subject.

I've already checked the FAQ, the Customization menu, "Learning GNU
Emacs" and searched the Google archives for this issue, but I can't
seem to find the answer. Any help would be appreciated.

-- 
Eric Pement