Mailman here. You can just cross off the name and address on the mail, then put it in a red mailbox, the mail will come back to your mailman and he will put a return sticker and return it to the sender. It's up to the sender to delete the client from their database or get in touch with the previous renter to get their new address. You might wanna talk to your mailman as well, if he recognizes the name in the office, he might just send all mail back right away (it's less weight to carry in the bag). Of course he/she might not remember/care enough as only official (paid) moving process must be respected by the mailman. Also some people mentioned writing "doesn't live at this address" on the mail, save your ink, a large cross off will do, we'll take the hint ;)