Error: There is still unsent mail...(due to design feature).

Error:

There is still unsent mail in the Outbox. You may want to send it the next time you use Microsoft Exchange.

Cause:

This message appears when quitting Microsoft Exchange and the Outbox contains unsent messages. Exchange does not give the user the option to send the mail prior to shutting down. This is working as designed.

Solution:

Re-open Microsoft Exchange and select 'Deliver Now' from the 'Tools' menu to deliver the unsent mail.