John Levine writes:
As other people have pointed out, a no-mail default is far more robust
than the current default, since a fair number of non-mail hosts turn
out to be running some sort of default SMTP server which will swallow
and lose mail. Or even if they aren't running an SMTP server, it can
take a week for the message to time out and bounce. It'd be much
better for such messages to fail immediately so the sender will
notice and can do something about it.
There is another argument: A fair amount of software constructs email
addresses, most notably
Sender: something(_at_)hostname
and
MAIL FROM:<something(_at_)hostname>
The best I can say for these addresses is that they're syntactically
valid. It would be a great joy to me if all the muckware would stop
assuming that blah(_at_)hostname is a deliverable email address.
Arnt