When sendmail forks a copy of itself to process a queue, it does so twice to prevent the creation of a zombie process. A zombie process is one that has lost its parent, and has not yet died. It continues to exist as though alive, yet it cannot be killed, hence it is a zombie.

This USE_DOUBLE_FORK compile-time macro is defined by default as 1 to enable the double fork to prevent zombies. In the rare instance that you are porting to a new system, you can redefine USE_DOUBLE_FORK like this:

APPENDDEF(`conf_sendmail_ENVDEF', `-DUSE_DOUBLE_FORK=0')

If you are running a precompiled sendmail binary, you can use the -d0.10 debugging command-line switch (-d0.10) to determine if USE_DOUBLE_FORK is defined (if it appears in the list, it is defined). New ports should be reported to sendmail@sendmail.org so that they can be folded into future releases.