My email address is being used by someone else to send SPAM
You are probably the target of email spoofing which is where a 3rd party cloaks their real email identity by using someone elses email address - yours in this case.
For more information on email spoofing, click here to read an article by Wikipedi.
In an attempt to combat this, SiliconDESIGNS has implemented a service called SPF or Sending Policy Framework on our servers, but this has only limited capability and impact. For more informaiton on SPF, click here and you will be taken to the official SPF website.
The reality is there is little you can do to prevent this from happening other than to assure your customers that you are NOT spamming them despite what they may think. You can of course refer them to them to the Wikipedia article to help them understand the problem better.
If you visit our home page, you will see a SPAM statement link which on reading, attempts to reassure people that we are most definatley not in the habit of spamming anyone. As you can see, all we are trying to do here is to reassure people of our integrity but other than that, there is not much else we ... or you can do.