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
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
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
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.