Mail glossary

67 postal terms, defined once and plainly. Each one is a short page rather than a paragraph buried in a guide, because the question behind them is nearly always the same: what exactly does this word mean, and does it change what I should send?

Proof & extra services

The add-on services that produce a record of mailing, delivery, or signature.

Adult Signature Required

A USPS extra service that requires the signature of someone 21 or older at the address before a piece is released, with no signature by anyone younger accepted.

Certificate of Mailing

A USPS receipt proving you mailed a specific piece on a specific date. It does not track the piece and it does not prove delivery.

Certified Mail

A USPS extra service that adds a dated mailing receipt and delivery tracking to a letter, with the delivery record kept by USPS.

Delivery Confirmation

A retired USPS service name for a basic delivery scan; today the same function is built into USPS Tracking on most mail classes.

Electronic Return Receipt

The digital version of the return receipt: instead of a green card in the mail, you get a PDF with the recipient’s signature image.

Insured Mail

A USPS extra service that reimburses the sender up to a declared value if a piece is lost, damaged, or has contents missing.

Proof of mailing

Documentation that a piece of mail was handed to USPS on a given date — the sending half of the evidence, not the delivery half.

Proof of service

A legal filing stating that a party was properly notified — usually a sworn declaration, supported by mailing evidence rather than replaced by it.

PS Form 3800

The USPS Certified Mail receipt form — the green-and-white strip carrying the tracking number that you keep as proof of mailing.

PS Form 3811

The paper return receipt — the "green card" that is signed at delivery and mailed back to the sender.

Registered Mail

USPS’s highest-security service: each piece is individually tracked and signed for through every transfer, and can be insured for high-value contents.

Restricted Delivery

A USPS extra service that limits delivery to the addressee, or to someone the addressee has authorised in writing.

Return receipt

A USPS add-on that sends the sender a copy of the recipient’s signature as proof the piece was delivered and to whom.

Signature Confirmation

A USPS extra service that requires a signature at delivery and keeps the record on file, without the mailing receipt Certified Mail gives you.

Mail classes

The USPS service levels a piece can travel under, and what each one includes.

Bound Printed Matter

A USPS mail class for advertising, promotional, or editorial material that is permanently bound — catalogs, manuals, directories — priced by weight rather than as a letter.

Business Reply Mail

A prepaid reply format: the recipient mails something back without a stamp, and the permit holder pays postage only on the pieces actually returned.

Every Door Direct Mail

A USPS saturation-mailing service that delivers to every address on selected carrier routes without a mailing list of names.

First-Class Mail

The everyday USPS class for letters and documents — typically 3–5 business days, with no tracking unless you add it.

First-Class Mail International

USPS's standard low-cost class for mailing letters, postcards, and flats to roughly 180 countries outside the United States.

First-Class Package Service

USPS's First-Class rate for small, lightweight packages, distinct from First-Class Mail, which covers letters and large envelopes.

Library Mail

A discounted USPS mail class for books, sound recordings, and other materials shipped between schools, libraries, and similar qualifying institutions.

Media Mail

A discounted USPS class for books and educational media only. Letters and correspondence are not eligible, and pieces may be inspected.

Parcel Select

USPS's ground shipping service for packages, typically used by high-volume commercial shippers who deposit mail deep into the USPS network.

Priority Mail

A faster USPS class — typically 1–3 business days — with tracking included as standard.

Priority Mail Express

USPS’s fastest tier, overnight to most addresses with a money-back delivery guarantee. paperplane does not offer it.

USPS Marketing Mail

The bulk advertising class, formerly Standard Mail: cheaper per piece, but it requires a permit, a minimum volume, presorting, and it travels slower.

Delivery & what goes wrong

Forwarding, holds, refusals, and where mail ends up when it cannot be delivered.

Ancillary service endorsement

A printed instruction on a mailpiece telling USPS what to do if it cannot be delivered as addressed — return it, forward it, or report the new address.

Change of address

The USPS request that forwards mail from an old address to a new one for a limited period after a move.

General Delivery

A USPS service that holds mail addressed to a post office itself, for pickup by someone with no permanent address or who is traveling, rather than delivering it to a street address.

Informed Delivery

A free USPS service that emails residents grayscale images of the letter mail due to arrive at their address that day.

Mail Recovery Center

The USPS "dead mail" office where undeliverable mail goes when it also cannot be returned to the sender.

Missent mail

Mail that was routed to the wrong destination by USPS handling error, as distinct from mail that is undeliverable because of a bad address.

Nixie

Postal-industry slang for a piece of mail that cannot be delivered as addressed, most often because of a bad or incomplete address.

Notice Left (PS Form 3849)

The slip a USPS carrier leaves when a piece requiring a signature could not be delivered, telling the recipient where to pick it up or how to request redelivery.

Refused mail

Mail that a recipient declines to accept at the point of delivery, which USPS then returns to the sender rather than leaving or re-attempting.

Return to sender

A mailpiece sent back to the sender because it could not be delivered, was refused, or went unclaimed.

Service standard

USPS’s published expected delivery time for a mail class — a target measured in business days, not a guaranteed date.

Undeliverable as addressed

USPS’s classification for a piece that cannot be delivered to the address written on it — the umbrella term behind most returned mail.

Vacant (mail status)

A USPS designation for an address the carrier believes is unoccupied, which affects whether mail is delivered, forwarded, or returned.

Addresses & data quality

The USPS data standards that decide whether an address is actually deliverable.

Address standardization

Rewriting an address into the exact form USPS uses — canonical abbreviations, directionals, unit designators, and ZIP+4.

CASS certification

A USPS certification programme for address-matching software, confirming it can correctly standardise and code addresses.

Commercial Mail Receiving Agency (CMRA)

A private business, such as a mailbox rental store, authorized by USPS to receive mail on behalf of customers at a street address rather than a PO Box.

Delivery Point Barcode (DPBC)

A USPS barcode encoding a ZIP+4 code plus a two-digit delivery point number, printed on mail to speed automated sorting to the exact address.

Delivery Point Validation

A USPS check that confirms an address is a real, individually deliverable delivery point — down to the apartment or suite.

NCOALink

The USPS-licensed dataset of change-of-address records used to update mailing lists before a mailing goes out.

Rural route

An older USPS addressing format (e.g. "RR 2 Box 45") used in rural areas before many were converted to standard city-style street addresses.

Secondary address unit designator

The abbreviation USPS uses for a unit within a building — Apt, Ste, Unit, Fl — placed after the street address to route mail within a multi-unit property.

ZIP Code

The 5-digit number USPS assigns to a geographic delivery area, used to route mail to the right regional and local sorting facility.

ZIP+4

The 4-digit extension USPS adds to a 5-digit ZIP code to identify a specific delivery segment — often a single building, floor, or even a large mail recipient.

Producing & sending mail

How mail gets printed, paid for, barcoded, and sent at volume.