Sometimes, no matter how good your address is, we still can’t deliver the mail. For instance, your customer may have moved or the building may be vacant. By using special addressing services called "ancillary service endorsements," you can give the Postal Service specific instructions for how to handle your mail if it is undeliverable as addressed. Depending on the purpose of your mailing, you may want those pieces forwarded to customers who have moved, or you may want a corrected address returned to you. Ancillary service endorsements include five basic phrases that are printed on the address side of your mailpiece:

Undeliverable mail is handled differently depending on the class of mail, the endorsement you use, and how recently your customer has moved. Some of these actions have fees associated with them and may cost you money.

Quick Service Guide 507 - Ancillary Service Endorsements[1]

First-Class Mail is forwarded to the new address free of charge and, if undeliverable, returned to you for free. That’s really good value for your postage. You can use an ancillary service endorsement to change how we treat your First-Class Mail. Undeliverable Standard Mail that doesn't have an endorsement is disposed of by the Postal Service. This is a good reason to make sure that your address list is correct and current.

TIPS
--Before you put an endorsement on your mailpiece, make sure you understand what service you will receive and what fees may be charged for that service.
--Pay special attention to the "weighted fee"[2] charge for Standard Mail endorsed "Address Service Requested" or "Forwarding Service Requested." This fee more than doubles the original postage cost and catches many mailers by surprise.
--The best way to avoid undeliverable or returned pieces is to check the accuracy of your address list[3] BEFORE you mail.

Delivery Address[4]

Return Address[5]

Additional Resources[6]