GLOSSARY
Mailer ID (MID)
By Justin Winter · Updated August 28, 2026
A unique number USPS assigns to a mailer or mailing agent, encoded in the Intelligent Mail Barcode to identify who sent a piece.
A Mailer ID identifies the business or service responsible for a mailing inside the Intelligent Mail Barcode's data, the way an account number identifies a customer. USPS assigns 6-digit and 9-digit MIDs depending on the mailer's volume.
It is infrastructure a mail-production system needs to register once, not something that appears on the letter itself or that a recipient would ever see.
See also
- Intelligent Mail barcode — The USPS barcode printed on letter mail that encodes routing and a unique piece identifier, enabling automation rates and tracking.
- Full-Service Intelligent Mail — A USPS program requiring mailers to register their Intelligent Mail Barcodes with a Mailer ID and unique serial numbers, in exchange for per-piece and per-container tracking data.
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