When a mail message is placed into the queue because of an error during the delivery attempt, the nature of that error is stored in the M line of the qf file. The error is usually prefixed with Deferred:

Deferred: reason

Delivery can be deferred until a later queue run because of a temporary lack of services. For example, the reason might be "remote host is down."

The form of the qf file M line is:

Mmsg

The M must begin the line. It is immediately followed by the msg with no intervening space. The text of msg is everything up to the end of the line. The msg created by sendmail can include the word Deferred:, followed by a reason. The envelope-specific M line should appear before the S line.

Beginning with V8.12, each recipient also has an M line preceding its R line.

If the msg is missing, sendmail simply prints a blank line rather than a reason when showing the queue with mailq or the -bp command-line switch. If the M line is missing entirely, sendmail prints nothing.

The maximum number of characters in msg is defined by MAXLINE in conf.h (MAX...). Prior to V8.12, there could be only one M line in a qf file.