What is an IMb?
The Intelligent Mail barcode is how USPS routes and tracks your mail — and a place where “looks right” and “is right” diverge. Drop a file for a free structural scorecard — no signup.
IMb detection + bar integrity on PDF · full field decode and presort checks on AFP (USPS module).
Every automation-rate letter in the US carries the same 65-bar symbol. Encoded inside, per USPS-B-3200: a barcode identifier, service type, your mailer ID, a serial number, and the delivery-point routing code — packed through base conversion, protected by an 11-bit frame check sequence, and expressed as constant-weight 13-bit characters interleaved across the bars. That pipeline is exactly why visual inspection is worthless: a bug anywhere upstream produces a barcode that is structurally perfect and semantically wrong. Honest validation reverses the pipeline and proves the FCS matches.
Frequently asked questions
What is the Intelligent Mail barcode?
The IMb is the USPS 65-bar, height-modulated barcode required on automation-rate letters and flats since 2013 (replacing POSTNET and PLANET). It carries tracking and routing in one symbol and powers Informed Visibility scan data.
What data is inside an IMb?
A 20-digit tracking portion (barcode identifier, service type ID, mailer ID, and serial number) plus a routing code of 0, 5, 9, or 11 digits (ZIP, ZIP+4, or delivery point).
Why do IMbs fail even when they scan?
The encoding pipeline (per USPS-B-3200) involves base conversion, an 11-bit CRC, and constant-weight character coding. An encoder bug produces bars that look perfect but decode to the wrong payload — which is why validation must reverse the pipeline, not just measure bars.
What happens if the IMb is wrong on a mailing?
Misrouting, lost Informed Visibility tracking, and at assessment time, lost automation discounts. Catching a bad encode before the mailing is the whole game.
How do I validate an IMb for free?
Drop a document into Preflight’s free IMb validator — it detects barcodes and validates structure in your browser session. The full USPS-B-3200 FCS round-trip and field decode run in the USPS module.