So I will preface by saying I do not have any "bank alts" and don't know how this will affect them. I dont see bank alts SENDING mail to non-account characters. This is strictly a suggestion as a leveler that has 3-4 gold selling ads in my mailbox every 1-2 hours.
Limit the amount of mail a < lvl 20 character can send to non-same account characters to 5 an hour.
Seems like a straightforward solution to this mail spam.