i had this similar but it was with league of legends and minecraft, i kept getting hacked even tho they have to have access to my gmail, and gmail check all the connections to the server and my emails and it only said my computer had accessed my emails, stopped after i changed my email password tho and formatted my computer. the only way i can think this is possible is that they used my computer as a relay to access my inbox and then send it from my computer to whoever requested it. other then that i have literally no idea.