Consider the following scenario:
You access a Microsoft Exchange Server 2010 mailbox by using Microsoft Office Outlook 2007 that has the Exchange email account configured in online mode.
The Outlook 2007 client has an additional transport or email account in the Outlook profile, such as POP, IMAP or Live Meeting Transport.
You send a deferred delivery email message.
In this scenario, the email message is not delivered, and remains in the Outbox folder.
To resolve this issue, install the following update rollup:
2579150 Description of Update Rollup 4 for Exchange Server 2010 Service Pack 1
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.