This of course only works in the 2 minute window where missed-message
emails are planned, but nonetheless likely avoids common cases of
emailing users with deleted messages.
Fixes: #3873.
Missed-message email replies using the reply-to of
noreply@zulipchat.com shouldn't advertise that "just replying" will
work.
Rebased and commit message rewritten by tabbott.
Fixes#3965.