Mail return

You're referring to a "Return Mail" or "Undeliverable Mail"!

Return mail, also known as undeliverable mail, is mail that cannot be delivered to its intended recipient due to various reasons such as:

1. Incorrect or incomplete address
2. Recipient has moved and left no forwarding address
3. Recipient has passed away
4. Mail is damaged or illegible
5. Recipient is not accepting mail at their current address

When mail is returned to the sender, it's usually marked with a "Return to Sender" or "Undeliverable" stamp. The sender may then choose to:

1. Resend the mail to the correct address
2. Update their mailing list with the correct address
3. Dispose of the returned mail
4. Use the returned mail as a reminder to update their records

In the digital age, return mail can also refer to emails that bounce back due to invalid or non-existent email addresses.