Two separate exchange servers defined in one Outlook 2010 profile (both accessed via RPC over HTTP).
Mailboxes receive email correctly. When you send email the following occurs:
The first account can send email fine. The second account's email gets queued in the Outbox.
If you close Outlook and re-open it the status of the two is reversed. The second email will send correctly and the first will simply queue the email in the Outbox.
This is happens with Outlook 2010 with SP 1 installed. It would be nice if this worked without odd workarounds.