SMTP server vs open relay

I sometimes use an external e-mail account besides my company account
to help keep mail sorted.

Our local SMTP server locked up and had to be rebooted.
As a result (don't ask, long confusing story) my IT admin had
me install IIS and setup a local SMTP server on my local machine.

To get it to work, I had to turn on relaying.  Now I can send and
receive e-mail and everything seems to be working fine.

Now I'm being accused of being a problem because I have relaying
turned on.  I have it set to only allow mail from my IP, so I think
that I'm safe.  Now I just need to prove it.
A tech from my e-mail provider tried to relay something through
my SMTP server and couldn't even see it, so he thinks I'm safe,
but I'd like something more athoritative that I could show the admin
to calm him down.

How do I prove that this SMTP server can't be used as an open relay?

-- 
Thanks,
Brad.