The 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!