Server Transfer problem

I'm wanting to server transfer my Death Knight from Khadgar - to Mannoroth, I'm ready to pay and do the process It just won't let me past the server selection phase.

I get this message

"The character you are trying to transfer has mail in his or her mailbox, or inbound mail that hasn't arrived yet (possibly from a quest). Please wait until all inbound mail has arrived and make sure your mailbox is empty before trying again. "

I have cleared my mailbox, there is absolutely nothing in there, and i've waited for 2 hours now

Could really use some blue advice!