The main() function should be written as just another function with a
return value. This allows for more reliable code reuse. Imagine that
main() grows too large and needs to be factored into multiple
functions. At that point, exit() is probably the wrong thing, yet can
also be hard to notice as it's in less-frequently-tested exceptional
cases.
Rather than *reall* sending mail here, we instead have a new test
program, smtp-dummy which implements (a small piece of) the
server-side SMTP protocol and saves a mail message to the filename
provided. This gives us reasonable test coverage of a large chunk of
the notmuch+emacs code base (down to talking to an SMTP server with
the final mail contents).
