Add an option that you can enable/disable to auto-return messages from people not in your guild lists or contact list (Friends list only, not followers since that’s one-sided). Also make it so if you have a full mailbox you can’t send more mail.
Not only will this prevent people from getting annoyed by gold sellers, it’ll also prevent the gold sellers from easily sending out more mail unless they clean up their mailbox.
And for the people who still do trades through mail, you could just not enable this, or add someone to your friends briefly…. you really should be using the TP though…