Just ran into this problem recently where the outlook mail client says "outlook has already begun transmitting this message"
When this happens the email can not be removed from the outbox. It also was never sent to the server. The solution I came up with was to go off line (disconnect network connection), close and restart outlook and then delete the message.
When this happens the email can not be removed from the outbox. It also was never sent to the server. The solution I came up with was to go off line (disconnect network connection), close and restart outlook and then delete the message.