hundreds of returned email warnings received

Our client suddenly started to received hundreds of email warnings, receipts, returned notifications etc. The notifications seem to come from a myriad of sources globally.

We first checked the PC for virus activity using NOD32 and Malwarebytes but found no infection. We checked all running processes and found nothing unusual.

Checking the date/time of the notifications shows that they continue 24/7 even when the PC is switched off, thus we have kind of ruled out the PC as being a source of SPAM.

We installed Spamfighter which removes about 1/2 the notifications automatically, which still leaves a huge amount to be deleted each day, but of course this is only masking the problem.

The email address is their main contact so it is "enquiries@xxxxxxxxx.org.uk" (xxxxxxxxx replacing their domain name) so I'm wondering if their email address is being used by a maliceous party.

I'm out of ideas what to do next to try find and halt the source, thus if anyone has any suggestion what to try next, it would most appreciated.
ensite31Asked:
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IMIronManCommented:
It is probable you are victim of of Backscatter:
http://www.spamresource.com/2007/02/backscatter-what-is-it-how-do-i-stop-it.html

Pay close attention to the area "If you are an Administrator"

Also, never post e-mail addresses on a webpage, neither viable on the page or in the source code.  Use a contact form and script.

Easiest way to stop it, if it is just the one e-mail address is to change it.  Probably not do-able since the client uses this in marketing materials???  

They're are many Third party services that you can run your mail through before reaching your mail server they may help.

You could spend much time and money "cleaning" this e-mail address, or ...
Make the changes to your server as suggested in the above link, then creates a new general delivery e-mail, and makes sure it doesn't appear on your client's website.

Another good article:
http://backscattervictims.blogspot.com/

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ensite31Author Commented:
Thanks IMIronMan, that's very useful information, appreciate your help.
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