sendmail program allows the developer to turn on
debugging and to print the queue from any remote site. This
capability is useful for solving occasional problems but opens a
potentially wide security hole.
In general, SMTPDEBUG should always be undefined. Later, when you
become more expert with sendmail, you might want
to have a standby version of sendmail ready (one
with SMTPDEBUG defined), just in case you need it.
There is no debugging switch that will let you know whether a
precompiled version of sendmail had this
defined. Instead, you must run sendmail with
-bs, then issue the SHOWQ SMTP command. If that
command displays the mail queue, that precompiled
sendmail was built with SMTPDEBUG defined, and
so you should not use it!