When we require TLS authentication on MTA, our Outlook 2007 clients cannot connect and send mail via SMTP. They get the following error:

"None of the authentication methods supported by this client are supported
by your server."

When I turn off TLS only and config the clients to no encryption then they connect fine.

All other clients (Outlook 2003, Thunderbird, Mac Mail, Mac Entrouage) connect just fine with TLS required.

Any thoughts on how to diagnose this?