Outlook IMAP send failures

[Klaus]

I have this issue where OL2007 (using an IMAP account) is not sending some
emails.

The biggest issue is that the messages go into the Sent Items folder with no
indication that they weren't sent.

I have tracked it back through Servers and there are NO connection attempts
from Outlook. This issue is intermittent with no common denominator in the
Addresses it has failed to send to (all out of the Contacts btw). It is
using SMTP Authentication.

Any thoughts?