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
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.
--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" 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 BEFORE you mail.