I have reproduced the behavior that Mickey identified on my Treo 700w.
Working with Jim Lawson, we've been able to perform some traffic analysis
and  have determined that Outlook mobile uses SSL (STARTTLS, actually) in
its initial connection, but no on subsequent connections.

 

This explains why it continues to work with the other SMTP providers that
Mickey tested, which also allow plaintext password authentication without
encryption, but fails with UVM's server, which requires an encrypted channel
to receive a plaintext password.

 

I've posted a description of the problem to an active Windows Mobile forum,
where a MS MVP has referred the problem to another expert. This morning, I'm
working on formally opening a support issue with Microsoft.

 

I'll post an update as soon as there's anything interesting to report.

 

--Geoff