Bill Frantz
I hope if such a system is set up, people who use it will realize that email is not 100% reliable. Just because algebra.com sent mail to toad.com, doesn't mean that toad.com actually received it. A small fraction of a percent of these messages will be lost.
Failing something catastrophic like a disk failure or a host down for more than a week, this should not be the case. Almost every site running sendmail has the Os ("SuperSafe") option set. That means sendmail will not respond to a "." at the end of a DATA command with SMTP code 250 until it has written the incoming message (and queue info) to disk and called fsync. Thus, you may get 2 copies of a message, but mail messages should not just disappear regularly at all, even if the network goes down or a machine crashes. Non-sendmail MTA's tend to be even more strict about this, not even allowing this behavior to be disabled.