Who applies the barcode to USPS letters with IMB
Executive summary
The barcode known as the Intelligent Mail barcode (IMb) is generated and applied by the mailer — that is, the sender or an agent acting for the sender (for example a presort house or print vendor) — not by USPS staff during processing, and printing is a prerequisite for automation pricing and tracking services [1][2][3]. The Postal Service provides technical specifications, registry steps (Mailer IDs) and tools, and its automated equipment reads the IMb once a piece enters the mailstream [4][5].
1. The technical and regulatory baseline: mailers must provide the IMb to get automation discounts
USPS rules make IMb usage mandatory for letters and flats prepared for automation prices and for many extra services; that policy creates a practical requirement that the entity seeking discounts — the mailer — must supply correctly formatted barcodes prior to entry into the mailstream [2]. The IMb encodes routing and tracking fields including a Mailer ID and serial numbers, so its content inherently identifies who produced and paid for the mailing [6][4].
2. Who actually prints the bars: the sender or an agent prints the IMb
Operationally the barcode is printed by the mailer, or by vendors the mailer hires such as presort houses, mailing houses, or commercial label/print providers; many USPS-facing guides and industry explainers explicitly state that the IMb is created and printed before acceptance, typically by the sender or presort house [3][7]. PostalPro and other USPS documentation frame container and mailpiece barcodes as “Mailer-generated” labels, underscoring that barcode application is a mailer-side activity rather than a step executed on the processing line by USPS employees [8][2].
3. Tools, certification and the Mailer ID: how mailers apply IMbs correctly
To print compliant IMbs, mailers obtain Mailer IDs and may use USPS-approved encoder/decoder tools, Business Customer Gateway services, or certified vendor software/hardware; USPS guidance and the Mailer ID application process show that the service expects mailers to generate the barcodes according to USPS specs [4][2]. Commercial providers such as postage platforms and print vendors advertise integrated IMb generation to eliminate manual steps for customers, and USPS offers validation tooling to confirm barcodes will scan in automated equipment [7][6].
4. What USPS itself does: reads and acts on IMb data, but usually does not apply it
Once mailpieces enter the system, USPS automated sorting and tracking equipment reads the IMb to route mail and provide visibility; sources repeatedly emphasize that postal machinery scans the IMb as part of processing [5][9]. Official materials about container labels and package barcodes treat barcode creation as mailer-generated, and the Postal Service’s role is acceptance, verification, and downstream scanning rather than routine application of IMb marks to individual letters [8][2].
5. Exceptions, vendors, and practical realities
There are practical variants: small-scale mailers can use integrated services from vendors (e.g., postage providers that print IMbs at the point of label generation), and some specialized USPS services (e.g., Origin IMb Tracing) have specific encoding rules that combine fields in different ways — but these remain mailer-side encoding choices or vendor implementations rather than evidence USPS applies the code at processing [7][4]. Documentation also makes clear that for Full Service IMb, uniqueness and certain fields are the mailer’s responsibility [3].
6. Limits of available reporting and alternative views
The reviewed sources consistently attribute IMb generation and placement to mailers or their agents; none of the supplied material states that USPS routinely prints the IMb onto letters as a standard operational practice. If claims exist elsewhere that USPS applies IMbs at acceptance for mailpieces lacking them, those claims are not covered in the provided reporting and cannot be verified here (p1_s1–[2]4).