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.