Beginning with V8.7, a SIGHUP signal will cause sendmail to re-execute itself with its original command line. This works only if it is running in daemon mode (with -bd, -bd). For example, consider initially running sendmail like this:

# /usr/sbin/sendmail -bd -q1h

Then imagine that you changed something in the configuration file and wanted the running daemon to reread that file. You could cause that to happen by killing the currently running daemon with a SIGHUP signal:

# kill -HUP `head -1 /etc/mail/sendmail.pid`

This will cause sendmail to execute the command:

/usr/sbin/sendmail -bd -q1h

The original daemon exits, and the newly executed daemon replaces it.

Be aware that this works only if you run sendmail using a full pathname. If you use a relative path, an attempt to restart sendmail with SIGHUP will fail, and the following warning will be logged at LOG_ALERT:

could not exec bad command line here: reason 

This is a very serious situation because it means that your original daemon has exited and no new daemon ran to replace it.