We received a report of bug which had supposedly been fixed in Exchange 2010 SP1, but which is apparently manifesting is the version we are currently running, Exchange 2010 SP2 RU6. We were able to confirm the existence of this bug by reproducing it. Description: This issue manifests with Outlook 2010 (and possibly 2007, but I have not tested this), if one has an Outlook profile configured with an online connection to Exchange and an additional mailbox connecting to an IMAP4 or POP3 server. (The specific examples we have encountered and reproduced are IMAP4 connections to Gmail.) Sending an email with an attachment from the Gmail account results in the send attempt failing, and the following error being thrown:

Additionally, the message which failed to send ends up in the Outbox of the Exchange mailbox. It is necessary to remove this item from the Outbox for other send attempts to work.

Workaround:

The problem does not manifest if the connection to Exchange is configured to run in cached-mode.

The Fix:

We’ve reported the issue to Microsoft Support, and they have responded that this is a known issue, and that it will be corrected in SP3 Update Rollup 1.  (We are already planning for an SP3 deployment, but currently have no ETA on the release of RU1 and the tentative release date for RU1 is May 11.)