The nature of e-mail permits anyone to generate an e-mail message that has your "From" address in it.  In other words, it's difficult to certify that the sender of an e-mail message as it appears in the mail headers is actually the person who wrote the message.

Spammers will often use legitimate e-mail addresses in their outgoing mail, either to lend credibility to the message, to avoid dealing with any "bounce" messages that might result, or both.  Some viruses will even use the address of the infected user in generating copies of itself to send to other accounts.

Aside from the identity verification issues, which can be solved by using more secure forms of communication such as encrypted e-mail, there is not much you can do to prevent someone from impersonating your address in junkmail messages.  

It's worth noting that it is possible that there is software on your website or on your local network that is allowing a malicious user to generate messages from you; you should make sure your network and desktop computing environment, and that any software you run on your website, does not allow this to happen.