We are having a similar problem here. The mail shows up in
the SENT folder but a while later we sometimes get an
e-mail saying the mail was not delivered. Many of the
sent e-mails, though, just end up in limbo. The strange
thing is that only a few people have this problem in an
office where everyone is configured the same - same
POP/SMTP server, same Outlook settings.
For some, changing the SMTP server seemed to help even
though it didn't make any sense. We don't know if it's a
timing issue or what.