When handling bounced mail, sendmail first tries
to deliver it to the envelope sender. If that fails, it next tries to
deliver to the user defined by the
DoubleBounceAddress option (DoubleBounceAddress). If the message cannot be delivered to that
user (perhaps because a valid user was not specified),
sendmail attempts to save the message to the
file defined by this DeadLetterDrop option,
usually the file /var/tmp/dead.letter.
The DeadLetterDrop option is declared like this:
O DeadLetterDrop=path configuration file (V8.10 and later)
-ODeadLetterDrop=path command line (V8.10 and later)
define(`confDEAD_LETTER_DROP',`path') mc configuration (V8.10 and later)
Here, path is the full path to the file
for saving unsaveable bounce messages. If path is
omitted, or if this entire option is omitted, no saving is performed,
and instead sendmail will log a panic and leave
the message in the queue, but renamed as a Qf
file. The default configuration is to not define this option. The
recommended value for path when defining
this option is /var/tmp/dead.letter (but setting
this option is not recommended).
The DeadLetterDrop option is not safe. If
specified from the command line, it can cause
sendmail to relinquish its special privileges.