Any postage applied to the envelope must be canceled by a usps employee. What does this mean?

Question

On the back of priority envelopes, it says, "any postage applied to the envelope must be canceled by a usps employee". What does this mean? Thanks

Answer 1

The stamp is marked with a time and place.

This of course is usually done by a machine (operated by a USPS employee), but can actually be done by hand with either a stamp or even just a pen with the employee's initials included.

This comfirms the posting and helps to prevent the stamp from being reused.

Answer 2

You know those black squigly marks over the stamp when you recieve a letter in the mail? Those are called cancellation marks.
Only USPS personel have the authority to run the mail through the cancellation machine or do it by hand stamping it.

Answer 3

In addition to the other answers, the cancellation stamp includes the time that the mail was posted, so in case of some controversy over when something was sent, or whose was sent first, there is a record.