mail-mode does not insert a date header

[florian@fsavigny.de (Florian v. Savigny)]

Dear sendmail.el users,

My gut feeling is am being stupid, and must be missing something, but
I hope you will forgive. (The info manual, at least, is silent about
this, and sendmail.el has not really revealed anything which
strikingly resembles the solution.)

A number of people have brought to my attention that my messages do
not contain a "Date:" header. (As should be the case with this very
message.) I can confirm this when I send messages to myself, and a
check of my outbox confirms that this started in August 2012. I have
no idea why.

I could of course use (mail-rfc822-date) to set the Date header in
mail-default-headers, but my feeling is that this is not how it is
done (and certainly not how it worked on my system until summer
2012). Also, this would have the problem of setting the date when I
start writing the message, not when I send it. (And sometimes, I store
drafts for quite a while.)

Can anybody tell me what I must be blatantly overlooking? Thank you so
much!

Florian