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A postmark is the inked cancellation stamped over your postage at a postal processing facility. It usually shows the facility's name and the date the mail entered the system. That stamped date is what courts and tax agencies treat as the mailing date, so a clear postmark can decide a deadline dispute.
| Type | What it looks like | Where it's applied |
|---|---|---|
| Machine cancellation | Wavy or straight lines inked across the stamp, with a dated circle beside them | High-speed sorting equipment at a processing plant |
| Hand cancellation | A round dated stamp pressed over the postage | Post office counters, often on request or for odd-shaped mail |
| Meter mark | A printed rectangle with the date and postage amount, no separate cancellation | Applied by the sender's own postage meter |
The classic postmark is the first kind: dark wavy lines "killing" the stamp so it can't be peeled off and reused, next to a circle naming the processing facility and the date. Its original job was fraud prevention. Its modern job is evidence: that dated circle is the postal system's own record of when it took custody of your mail.
Because the postmark is applied when the mail reaches the processing plant, which is often the day after you dropped it in the box. The pickup schedule posted on the collection box tells you the last collection time; the postmark tells everyone else when the system actually took over. For anything with a deadline, that distinction is the whole game.
Tax filings are the famous example. Tax agencies generally treat a return as filed on time if the envelope bears a timely postmark, even when it arrives days later. The same logic shows up in rent disputes ("the check was mailed on the 1st"), contest and rebate entries, and court filings with mailing deadlines. When the date is contested, the postmark is the piece of evidence both sides reach for, because it was made by the postal service, not by either party.
Sometimes nothing is wrong. Mail with metered postage or a pre-printed indicia doesn't get a traditional cancellation: the date is printed by the sender's meter, which is exactly why it's weaker evidence than a postal cancellation. The sender controls the meter's date, so a meter mark proves less. Some presorted business mail skips the classic postmark for the same reason.
But a stamp with no cancellation at all can be a clue worth noticing. It may mean the piece hasn't been through a processing plant yet: your mail could still be sitting in a collection box or on a truck. If a deadline matters and you want proof of when you mailed something, don't rely on the postmark alone. Get a certificate of mailing from the post office counter: it's a receipt stamped with the date you handed the piece over, and it doesn't depend on what happens downstream.
Drop mail before the last collection time posted on the box, not after. Mail dropped after the final pickup waits for the next collection, and its postmark will carry the later date: a 5:05 drop into a 5:00 box is tomorrow's mail. For something truly deadline-critical, hand it to a clerk at the counter and ask for it to be cancelled by hand in front of you. And never backdate a postage meter to manufacture a timely date: a meter date is only as honest as the person who set it, and discrepancies get noticed when money is on the line.
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