Undeliverable Messages

If you specify an incorrect user address when you send a message, the system responds with the message

user@machine...User unknown

The message is then returned to your mailbox. The next time you type the mailx command, the header states that you have returned mail, similar to the following example:

N 1 Mailer-Daemon Fri Jan 3 11:13 8/49 Returned mail: User unknown

When a message cannot be delivered, the file is also copied to a file in your home directory named dead.letter.