Error Handling

When mail can't be delivered, the mailer catches and checks the status code, and a suitable error message is given as appropriate. The exit code must conform to a system standard. If a nonstandard exit code is used, sendmail transmits the message, "Services unavailable".

Queueing for Retransmission

If the mailer returns a "temporary failure" exit status, the message is queued. A control file describes the recipients and various other parameters. This control file is formatted as a series of lines, each describing a sender, a recipient, the time of submission, or some other parameter of the message. The header of the message is stored in the control file so that the associated data file in the queue is just the temporary file that was originally collected.

Return to Sender

If errors occur during processing, sendmail returns the message to the sender for retransmission. The letter can be mailed back or written to the dead.letter file in the sender's home directory.