check_smtp not working when scheduled

I am trying to set up a new check in Nagios 3.5.0, making sure that my mail server is contactable on port 25 over the internet.

I am using the check_smtp command. When I run this command from the command line I get this:

[root@cwp-nagios-03 libexec]# ./check_smtp -H mail.****.com
SMTP OK - 0.068 sec. response time|time=0.068033s;;;0.000000

This looks like everything working correctly, so I added a new service definition:

define service{
    use                     generic-service
    host_name               CL_WEB_CHECKS_DR
    service_description     Mail Server
    check_command           check_smtp!mail.***.com
}

When this command is run by Nagios, I get the following result instead:

Status Information: Connection refused
SMTP CRITICAL - 0.001 sec. response time
Performance Data:   time=0.001362s;;;0.000000

I am a bit lost as to why this is happening, so any help would be great!

Also, I am sure you will want to see it, so here is the command definition for check_smtp:

# 'check_smtp' command definition
define command{
    command_name    check_smtp
    command_line    $USER1$/check_smtp -H $HOSTADDRESS$
    }