The following response is quoted from 'Ask Leo' website ...

This is spam, pure and simple.  Abusing your email address is only one of many techniques spammers use to throw their garbage into our mail boxes.

What you're seeing is called "spoofing" or more correctly "From-spoofing" - sending email that appears as if it's coming "From:" someone that its not.

Spoofing is a technique that is used in just about every bit of spam you see today. Spammers are trying to hide where the email comes from and are doing so very effectively.  The From: address is meaningless on spam - it tells you absolutely nothing.  It requires more detailed analysis of the email headers, and even then at best you might be able to get the IP address of the computer sending the email.  As I've discussed ad nauseam, the IP address is pretty much useless to you and me.

The fact that you're seeing your email address used in the "From:" field shouldn't alarm you.  It might be annoying, but there's no need to worry about it.  You're already on spammers lists to get spam and they're using that same list, or variations of it, to select which addresses to use when spoofing.  And there's currently no effective way to stop them from spoofing.

When you see your own address spoofed in the From: field of spam, it's happening for one of two reasons:

• They're trying to spam you, and know that it's unlikely you'll block email from yourself. In fact, as you've seen, it's not even always possible, but I'd consider it a bad idea even if you could do it.  It'll prevent certain types of legitimate email from reaching you.

• They're trying to spam someone else, and what you're seeing is a bounce message indicating that the original spam was rejected by its intended recipient. Since the email looks like it came "From:" you, you get the bounce message.

"But what do I do?"

First, realize there's nothing you can do to prevent From-spoofing.  Eventually your email address is going to show up in the "From:" field of spam that you had nothing to do with.  In fact as you've seen, it probably already has.

The only thing you can do is to keep doing whatever it is you do to control spam.  I run two levels of filters, and even then some gets through.  It's a juggling act because I don't want to risk marking something as spam that isn't - hence a little more spam gets through. 

Depending on your mail program, you may have similar or additional options available.  We offer server spam filtering to add to your own 'local' spam filter(s).

For more information on the spam filters available here, see these articles:
http://support.realwebs.com/KB/a5/spam-low-spam-med-spam-high.aspx
http://support.realwebs.com/KB/a18/how-to-spam-filtering-user-account.aspx