The -d99.100 debugging switch^[8] prevents the sendmail daemon from forking and putting itself into the background. This leaves the running daemon connected to your terminal so that you can see other debugging output. For example:

^[8] There is no -d99.1 information available.

# /usr/sbin/sendmail -bd -d99.100 -d9.30

This allows you to watch the daemon perform RFC1413 identification queries when SMTP connections are made. See also -d52.100, which prevents sendmail from disconnecting from its controlling terminal device, or the -bD command-line switch (-bD), which does both.