Emails ‘sent’ from Blog via Outlook Express

For the past couple of weeks I have been getting no less than twenty emails per day that were of the “Mail Undeliverable – Returned to Sender” variety. I neither recognize the email addresses nor are they in my address book (the sent to address).

I have my blog at motorcyclebloggers.com and my email for that domain name is set up through Outlook Express. The outgoing sender address (me) is from “(4-6 random letters)@motorcyclebloggers.com”

Is this a WP thing, a PC thing, or a server thing?

I have the latest McAfee, Adaware, and Spybot and all are clear.

Thanks