As of lately we have had a few users where they compose a message in outlook and when they hit send the message goes to their outbox to send but it remains there and nothing you do will release it. However when the user logs into Outlook Web Access the message sends perfectly fine. This immediately indicates that the issue is client side in Outlook.
I must have spent ages trying to get Outlook to send but eventually a colleague of mine find a setting deep within Outlook which fixed the issue of emails not being sent and stuck in the outbox. The setting is as follows:
Outlook -> Tools -> Options -> Preferences (Tab) -> Email Options (Button) -> Advanced Email Options… (Button)
Then when there make sue that the follow tick box is un-checked: “In folders other than the Inbox, save replies with original message”.
Not sure what this does but when we un-tick it mail flow starts working again.