R1:72e33c9e5a53

Fix a threading issue with Amazon SES

Summary:
Amazon SES does not allow us to set a Message-ID header, which means
that threads are incorrect in Mail.app (and presumably other applications
which respect In-Reply-To and References) because the initial email does not
have anything which attaches it to the rest of the thread. To fix this, never
rely on Message-ID if the mailer doesn't support Message-ID.

(In the Amazon SES case, Amazon generates its own Message-ID which we can't
know ahead of time).

I…