Why don't all USPS letters have the IMb barcode printed on them
Executive summary (2–3 sentences)
The Intelligent Mail barcode (IMb) is a 65-bar, sender-applied code that combines routing and tracking data and can be required for automation discounts or for Full-Service reporting, but its use is optional for many mailers and constrained by technical, programmatic and physical rules (PostalPro; USPS tech guides) [1][2]. Practical barriers—size and placement limits, encoder and printer requirements, Mailer ID and serial-number rules, and selective USPS program mandates—explain why not every letter rolling through USPS carries a printed IMb [3][4][2].
1. The official rulebook: IMb is powerful but not universally mandatory
USPS documentation and technical specifications make clear that the Intelligent Mail barcode is the standard symbology for combining routing and tracking information, and that use of IMb became the way to qualify for many automation prices (the IMb specification and USPS guidance) [1][5]; however, the IMb remains optional for many classes and uses except where USPS program rules explicitly require it, meaning there is no blanket legal requirement to print an IMb on every piece of mail [2][6].
2. Programmatic triggers: who must use IMb and when
Certain USPS programs and pricing incentives force IMb adoption—Full-Service and automation-price eligibility require properly encoded IMbs and related participation (PostalPro; USPS guides) [1][2]—while other mailers can opt for non‑automation options or omit fields for simple tracking use. The package version (IMpb) is mandatory for many commercial parcels, underscoring that requirements vary by product and entry channel rather than applying uniformly to all mail (PostalPro; Pitney Bowes) [7][8].
3. Technical and production limits: size, placement, encoders and fonts
Generating a compliant IMb is not just “print and stick”: the barcode is a 65‑bar, four‑state construct that requires encoding software or fonts, and USPS documents state an encoder is required to convert numeric fields into the bar pattern (USPS technical guides) [2][4]. Mailpiece dimensions and barcode clear-zone rules also limit where IMbs can legally and reliably be placed—letters and flats must meet minimum/maximum size rules and placement tolerances—so small or unusually sized pieces often cannot host a compliant IMb (PostalPro; Compu‑Mail) [1][3].
4. Data requirements and exceptions: Mailer ID, serial numbers and special cases
An IMb normally must include a valid Mailer ID (MID) and serial number to uniquely identify pieces, and USPS specs spell out exceptions—Origin IMb tracing combines fields or allows different rules, and the Barcode Identifier (BI) field is usually “00” except for specific flat/OEL situations—creating administrative steps for mailers that some choose to avoid (USPS-B-3200; Steps to creating IMb) [4][9][10]. Smaller senders or reply‑mail operations may not have or want to manage MIDs, unique serial sequences, or the additional setup required to meet Full‑Service reporting.
5. Cost, complexity and vendor dependence: why smaller senders skip IMb
Although USPS and industry sources highlight operational and tracking benefits to IMb (PostalPro; Pitney Bowes), the reality is that adopting IMb can require new software, certified printers, and process changes; USPS guidance recommends testing or working with certified vendors, signaling a barrier to DIY printing for many small businesses and individuals [1][8]. For low-volume or ad-hoc mailers the compliance overhead and up‑front cost can outweigh discounted postage or tracking benefits, so they send without IMbs.
6. What the trade press and USPS emphasize—and what they don’t
USPS and industry guides emphasize that IMb consolidates services, enables Full‑Service analytics and is required for many automation prices, while also documenting normative exceptions and technical tolerances (PostalPro; USPS tech resources) [1][2]. Reporting that focuses on the IMb’s benefits can underplay the practical limits—size, machine‑print requirements and mandatory data fields—that explain why many letters still arrive without an IMb; the primary agenda of USPS documentation is operational standardization and incentivized adoption, not universal compulsory printing [2][11].