This one particular user is set up slightly differently to everyone else. Shes a PA to the CEO - call her A.
She logs in to the network as A and has her own mailbox. However, she full access to CEO mailbox and user B.
Her outlook is set up as default CEO, with A and B also. She rarely uses her own A mailbox.
Most of the day she SENDS AS CEO.
Intermittently, emails get stuck and remain in the outbox. Sometimes if you go into email and send again it works (but leaves copy behind in outbox). Sometimes not.
Also, new emails SEND AS CEO go into CEO SENT items. Emails replied to (even though SEND AS CEO) go into user A SENT items.
Tried new profile. Still the same.