How is ‘Postage Due’ Domestic Mail handled?

Throughout its journey, mail may be periodically checked to ensure it has proper postage. If a mailpiece does not have proper postage, it may be returned or delivered according to the following scenarios:

If the item is without postage, or it appears postage fell off; it is endorsed "Returned for Postage Mail" and sent to the return address on the mailpiece.

Please Note: The USPS®u makes a special effort when the mail piece has the appearance of a greeting card. Rather than being routed to the Mail Recovery Center because of no postage or below minimum size, greeting cards without a return address will generally be delivered with postage due during the month of December. (This is for domestic First-Class Mail® only. Foreign greeting cards without postage go back to the mailer for proper postage.)
If the item is short paid, it will be delivered postage due and the letter carrier will endorse it to show the amount of the postage due.
The recipient must pay incash for postage due mail before the mail is delivered
Depending on location, letter carriers may or may not be able to collect Postage Due directly from a customer or a customer's mailbox; for further information on how to pay for the Postage Due, please contact your Local Post Office facility
Rural carriers are not required to go to the door with a postage due letter. Policy stipulates that rural carriers go only as far as the mailbox and blow the horn. The recipient must come out to the mailbox to pay and receive the letter.

Note: If the shortpaid item cannot be delivered as addressed (including if the recipient refuses to pay for and accept it), it is endorsed "Returned for Additional Postage" and sent to the return address on the mailpiece.

A returned shortpaid mailpiece can have the necessary additional postage affixed (leaving the "postage due" message intact) to the original piece and does not have to be placed in a new envelope or wrapper.
If an item is returned because of no postage or insufficient postage and one of the following is the case:
No return address is shown; or
The delivery and return addresses are identical; or
The delivery and return addresses are different, but for the same person or organization; then

the mailpiece is treated as dead mail and goes to the Mail Recovery Center in Atlanta, GA. (For additional information on the Mail Recovery Center, go to the USPS Mail Recovery Center (MRC) FAQ)