I have a strange problem - I have a user account that I can send email from in Outlook and the email appears in the Sent folder but never arrives at the destination nor does it show up in Message Tracking. If I send a message from this account using OWA the message arrives at the destination and shows up in Message Tracking.

Hi, Please try to use outlook in safe mode without any add-ons. Besides, please try to configure the user account on other workstation and then test the issue. I suspect it could be the client side issue, sometimes anti-virus application may also cause the issue. Regards, Xiu