Doesn't work with quest bot, mailing is bugged for both grind and quest bot, it looks like its using vendor as mailbox...
Thing is when bot goes to vendor it sells items, than instead of moving to mailbox it "sends" mail (it doesn't send just it logs it as sent) and when you move manual to mailbox (don't know if you need to open it) it will start sending but not from start it will continue sending from time it sold items to vendor till time you manually moved to mailbox and you will end up with half or more items in your bag that will be counted as sent afterwards bot will continue moving as if your items were sent and inventory is empty...
Edit:
Maybe someone could create plugin that when you come to desired lets say Dalaran it will force move to nearest mailbox/coordinates?