I am using check_tcp to monitor smtp service. One of my server under monitoring sometimes respond to check_tcp -p 25, but sometimes not.
Can anybody tell me how this command works? I can tell it is different from telnet x.x.x.x 25.
You have to locate the executable check_tcp file. It is usually somewhere under the /usr directory. After you have located it, run form the terminal:/path_to_executable_plugin_file/check_tcp --help
There is also check_smtp plugin:nagiosplugins.org/man/check_smtp