Open relay? pls help

Open relay? pls help

Hi all

I am recently having problems where two particular addresses which are using
our company domain name are sending e-mails to outside addresses and in
return we are receiving a lot delivery failure reports. We have checked if
we are an open relay and from the websites it doesnt look like.

Anything else we can do to confirm? we also did the telnet, ehlo test etc..
to confirm and the 550 code was returned.

Your help is very valuable to us