Simon Josefsson writes ("Re: sendmail.el bug or expected behavior?"):
I suspect that if the MTA is trying to deliver the message to its
final destination, then it may hang regardless of what MTA is used.
IMO the correct way for a MUA to invoke sendmail (on Debian) is
/usr/sbin/sendmail -odb -oee ...
(To be portable to non-FHS systems, use /usr/lib/sendmail)
This is supposed to
(a) queue the message, exiting non-0 if unsuccessful
(b) fork off a detached delivery attempt, which will
deliver the message, leave it queued, or bounce it,
as appropriate
(c) exit 0, leaving the delivery attempting going
The problem that RMS mentions happens if you don't give any -odX
options: usually that means you'll get `foreground' delivery, which
typically means that the MTA will try one delivery attempt now.