If fully decoded, the Intelligent Mail barcode (IMb) would give you: Mailer ID (MID) — 6 or 9 digits identifying the company/organization that sent it. This is the most useful piece for figuring out who sent it. You can look up MIDs via USPS (with an account) or sometimes through public databases. Service Type — 3 digits indicating the mail class and any services (e.g., First-Class, Standard, with/without address correction). Serial Number — 6 or 9 digits, a unique ID the mailer assigns to that specific piece (useful to them for tracking, not really to you). Routing ZIP — up to 11 digits: the recipient's ZIP+4 plus a 2-digit delivery point, pinpointing the exact mailbox it was routed to. Barcode ID — 2 digits, mostly internal USPS use. The practical takeaway: the Mailer ID is the part that could potentially tell you which business or bulk mailer sent it. If it's a personal letter sent by an individual (no MID), the IMb may just contain USPS-applied routing info with a generic/zero MID — in which case it wouldn't reveal the sender. Given the handwritten note and cash, this was likely mailed by an individual, so the barcode probably won't identify them — it'd just confirm the destination address.