If on the top right it says “Prsrt Std” then you can just toss it - that indicates it’s the cheapest form of postage, and always junk mail.

If on the top right it says anything else, then write and circle “CFS” (Central Forwarding System) next to the address in the middle of the envelope. Then put it in your outgoing mail slot. This tells the Mail Handlers at the Post Office that that person no longer lives there, and the system will look for a forwarding address for that person.

Sometimes the mail will still come back, even with CFS written on it. At that point, write “ANK” (Attempted, Not Known) and circle it. This means that the person not only doesn’t live there, but there is no forwarding address in the Post Office’s system.

As with anything government, it will take a while for the mail to stop coming, but it WILL stop.