If your former roommate failed to inform the United States Post Office that he moved and you don't want his mail piling up, you don't need to wait for him to pick the mail. If you have his new mailing address, it just takes a few minutes to reroute the mail to his new location.
Cross out the address on the envelope using the ink pen. Do not cross out the recipient's name.
Print the new mailing address on the envelope, as close to the old address as possible.
Write "Please forward, not at this address" on the same side of the envelope as the recipient's name and address.
Put the mail back in your mailbox or drop it off at the Post Office.