Oops! Mail delivery retries to the rescue :).

Aside from an extremely exciting hour where I accidentally created a suppression for my own address and could not test anything, this went reasonably smoothly.  (Those who might want to help us out with ops: we now exclusively use exim for routing messages to mailman, and maybe delivering some emails from Trac; it no longer sends messages to users.  So we could maybe make a Docker container out of that.)

-glyph