From my hosting company:
Unfortunately it is trivial for anyone to use your email address as the sender (from) address for mail messages. They don't need to send the mail via our servers, and there is little you can do to stop them. This is happening more and more because most ISPs now block mail that doesn't come from a valid address, and many mark as spam mail from obviously fictitious addresses at hotmail.com, yahoo.com, etc. Consequently many spammers now find domain names from the WHOIS database and use these to make plausible looking 'from' addresses for the email they send.
In addition, many of the current viruses use fake 'from' addresses in the virus infected mails they send out once a machine has become infected. Usually they use addresses gleaned from the computers address-book and recently received emails, so if you are receiving virus infected mails, you might find they are coming from someone you know.
This can become a serious problem when spammers use your address as the 'from' address in bulk spam email, in that you may receive many bounce messages from any invalid addresses they send the spam to. Unfortunately it isn't easy for us to block this mail, but as the from address on the mail will probably be a fictitious recipient at your domain, you might be able to block these coming to you by not using the 'postmaster' (catchall) recipient, but this is only an option if you have a forwarding or hosting account.
The point is that if you if you use a "add sender to block"list, the email address that you might block could be a genuine Halifax addres