This plugin is part of Official Nagios® Plugins package and its documentation be found here.
Example of use and return of command
You must access the command directory before running it trough the following path: /usr/local/opmon/libexec/
[root@opmon]# ./check_smtp -H IP_DO_HOST -w 10 -c 20 SMTP OK - 0.002 sec. response time|time=0.001568s;10.000000;20.000000;0.000000