Faction Change / Mail in Mailbox Error

I was attempting to do a faction change for my Character Tboneboom on Frostmourne and it says that I have mail or that mail is en route to my mailbox.

Error Message:

" An error has occurred on this page. A character cannot change factions while there is mail in their mailbox or en route to their mailbox."

I logged in and deleted the saved mail in my mailbox and cancelled all my auctions and to my knowledge there is no mail on the way. I even waited a few hours to see if any mail arrived, but none has come through. I've tried the faction change process a few times now over the past few hours and still keep getting the same error message. Is there a way to fix this please?

Your help in resolving this issue is greatly appreciated.