This is normally down to 1 or 2 things:

1. A virus on your local machine, we recommend that you do a full system virus scan.

2. The longer your domain has presence on the Internet, obviously the more people recognize it, unfortunately so do spammers. As there is a general flaw in the the way that sending emails work on the Internet anybody can send as anybody else's email address if they have the technical knowledge. So what is probably happening is that spammers are sending emails using your addresses and you are receiving the bounce backs from the failed email addresses, theres not really anything you can do to stop them sending as your email addresses however there are a couple of things you could do to reduce the emails that bounce back. If you have a catchall email address that accepts ANY mail for your domain we can reduce this to only accept certain aliases such as bill@ fred@ etc.. If you do not have a catchall account and you are a fairly old customer it may be worth speaking to your account manager at SiteWizard regarding migration  to one of our newer email servers with spam filtering facilities.