Unread Notification to Non-Existant User..

I had a user that gave a bogus email address and I was unable to find their correct email address. So, I deleted the account. However, they had a private message that was still trying to be delivered. I removed the message from the DB. However, every day the site tries to send the non-existant user an email. The email is not addressed to anyone and my server simply forwards the failure to me. Does anybody have an idea of what's triggering Private Message to generate the email for the non-existant user? I would think there's something still left in my DB that's causing it.. but I'm out of ideas on where to look. What triggers the message to be sent? Any help would be HUGELY appreciated. Thanks.