How a letter gets scanned
When you drop a letter in a collection box, nothing scans it there. A carrier picks it up and takes it to a processing plant, where machines face, cancel and sort it, reading the address and the barcode as they go. Each pass through a sorting machine records a scan for that barcode with the time and the plant's location. The letter is sorted again at the plant that serves the destination, and once more into the exact order of the carrier's route.
This is a letter that left Austin on a Friday evening, as PigeonTrail shows it:
- Austin, TXIn the mail
- Coppell, TX
- Memphis, TN
- Indianapolis, IN
- Columbus, OHAt the plant near the buyer
It went through Coppell and Memphis on Saturday and Indianapolis on Sunday, since USPS's plants sort mail on Sundays even though carriers don't deliver, and reached the plant near the buyer on Monday morning.
The scans in order, and what each one means
| The scan | Where it happens | What it means |
|---|---|---|
| First plant scan | The first plant, usually near where it was mailed | USPS has the letter and its machines have sorted it, which PigeonTrail shows as in the mail. This is the first scan most letters get, often the evening of the day they're picked up. |
| Scans along the way | Plants along the way | The letter was sorted again on its way across the country. Short trips may have none of these. |
| Destination plant | The plant that serves the destination | It has arrived in the destination area and is waiting to be sorted for a carrier. PigeonTrail shows it as at the plant near the buyer. |
| Sorted for the carrier | The destination plant | It was sorted into the order of the carrier's route, usually the night before delivery. This is the closest a letter gets to "out for delivery." |
| Delivered | The carrier's route | USPS's estimate that the letter was delivered, recorded when the carrier's scanner leaves the area of the address's ZIP+4 that day. |
USPS's package tracking uses phrases like "Arrived at USPS Regional Facility" and "Out for Delivery." Those come from package scans, which a letter doesn't get. For a letter, a plant scan is the same idea as a regional facility arrival, and sorted for the mail carrier is the same idea as out for delivery.
Is “delivered” certain?
No. USPS's guide to this data calls a letter's delivered event "an implied delivery," with the operation code 517. Nobody scans a letter at the mailbox, so USPS infers delivery when the carrier's handheld scanner leaves the area of the address's ZIP+4 on the route that day. For a PO box, it comes from a scan the post office makes once its boxes are filled.
Most of the time the letter is in the mailbox when it says delivered. Now and then the event shows before the letter is really in the box, for example when it is held for the next day. USPS's Domestic Mail Manual says its letter tracking service "does not provide a delivery scan or proof of delivery and not every piece is guaranteed such data or complete data." So PigeonTrail shows the event as "USPS marked it delivered" and never calls it proof.
Why a letter has no scans
- It hasn't reached a plant yet. A letter picked up late on a Friday or Saturday may not be sorted until the next evening.
- It was nonmachinable, too stiff, square or lumpy, so it was sorted by hand and never passed a barcode reader. Our nonmachinable guide lists the causes.
- The barcode couldn't be read, because the print was faint, smudged, covered by tape, or sat outside an envelope window.
- It went to the local post office without full machine sorting. A 2018 USPS Inspector General audit found that letters are sometimes sent to the local delivery unit "as raw mail that has not been sorted to delivery point sequence," the order of the carrier's route.
A letter with no scans is usually still on its way. Give it until a day past the date USPS expects it before worrying. The delivery time tool shows that date for any two ZIP codes.
Why a letter skipped “delivered”
USPS's own table of delivery areas marks 7,236 of 41,758 ZIP codes, about 17%, as zones where mail is sorted by hand at the last step and "no tracking code scans should be generated." Letters to those areas get their plant scans and then go quiet, often with no sorted for the carrier scan and no delivered event, while arriving as usual. The same can happen when the carrier's scanner doesn't report the route that day. That is why PigeonTrail looks at the day USPS expects a letter, and the time since its last scan, before it calls a letter late.
Questions
What does “arrived at USPS regional facility” mean for a letter?
That phrase comes from package tracking. For a letter, the matching event is a scan at a USPS processing plant, which means its machines sorted the letter there.
Why does my letter have no scans?
Usually because it hasn't reached a plant yet, it was sorted by hand because it was nonmachinable, or its barcode couldn't be read. Most letters with no scans still arrive.
Why did my letter skip delivered?
About 17% of ZIP codes are areas where USPS sorts the last step by hand and generates no scans, and a carrier's scanner sometimes doesn't report. The letter can arrive normally without a delivered event.
Is a delivered scan on a letter certain?
No. USPS calls it an implied delivery, recorded when the carrier's scanner leaves the address's area on the route. It is not a scan at the mailbox and not proof of delivery.
Sources
- USPS, Informed Visibility Mail Tracking and Reporting User Guide, version 2.19.9, September 2026, Appendix E.3.2: postalpro.usps.com
- USPS, Domestic Mail Manual 507.10, Informed Visibility: pe.usps.com/text/dmm300/507.htm
- USPS, Automated Zone Indicator table, file of September 5, 2026: postalpro.usps.com. The 17% share is our count of the ZIP codes in that file.
- USPS Office of Inspector General, "Intelligent Mail Barcode Costs for First-Class Mail Letters", CP-AR-18-007, September 25, 2018: uspsoig.gov