check_mail_system shows Warning

Strange thing : if I launch this plug-in from console, it works perfectly and says that message was delivered in 6 seconds.
But it nagios web interface itself, it is producing warning, like:
Notification Type: PROBLEM

Service: MXLoop test
Host: ********************-Loop-Test
Address: *****************
State: WARNING

Date/Time: Mon Feb 1 05:26:55 EST 2010

Additional Info:

(null)

What can be the reason?