Meanings of be no postmark in English

There was no postmark on the envelope, which leaves me wondering when they were sent out.

With tears welling up in her eyes, she said that she had to call the police because there was no postmark.

"Here is a letter," she said: "there 's no postmark.

There was no postmark on it, no name or return address, no sense of where it had come from or who had delivered it.

The other birthday tokens, you received, Mademoiselle Bertha, I always contrived to send you by private hand; thus, there was no postmark to awaken suspicion.