The check_smtp plugin can be used for monitoring your email servers. The commands.cfg file contains a command definition for using the check_smtp plugin, which looks like this:
define command {
    command_name    check_smtp
    command_line    $USER1$/check_smtp -H $HOSTADDRESS$ $ARG1$
}
A simple service definition for monitoring the SMTP server on remotehost would look like this:
define service {
    use                 generic-service     ; Inherit default values from a template
    host_name           remotehost
    service_description SMTP
    check_command       check_smtp
}