A returned mail message

I've just received the following message and had another like it recently.
There was an attachment too - which wasn't a message I sent. (I deleted it
without reading in case virus.) I don't recognise the address below.

Does this mean someone is sending sp@m made to look as if it's coming from
our list? I'm concerned that ISPs may begin to ban messages from our domain
if it's apparently sending sp@m messages.

Is there anything I can do about it?

Thanks

Clare

-----Original Message-----
From: Mail Delivery Subsystem [mailto:[email protected]]
Sent: 26 April 2006 13:11
To: [email protected]
Subject: [-SPAM-] Returned mail: see transcript for details

The original message was received at Wed, 26 Apr 2006 13:10:32 +0100
from root@localhost

   ----- The following addresses had permanent fatal errors -----
[email protected](reason: 553 sorry, relaying denied from your location
[67.15.98.3] (#5.7.1))

   ----- Transcript of session follows -----
... while talking to mailstore1.secureserver.net.:
>>> DATA
<<< 553 sorry, relaying denied from your location [67.15.98.3] (#5.7.1)
550 5.1.1 [email protected]... User unknown
<<< 503 RCPT first (#5.5.1)