Since there are problems with having Emacs check at run time
whether there is a configured MTA, I propose doing it inside
`make install'.
It could run a program that tries sending messages to certain chosen
addresses, and detects the results. This program could afford to wait
a certain amount of time for the results to show up, with no danger it
will be terminated by a user who does not realize it is still working
on the problem.
Since `make install' is run as root, it could look for a bounce
message in root's local inbox too.