On my wifes 8520 it seems as if every time she has got an email in the past the BB has saved the email address. These addresses do not show up in the contacts on the phone, web or her computer just in the BB.
For example, say she goes to enter an email to Donna, as she types "d o n" a donotreply email address will be displayed as an option. She only has 17 contacts in her address book and I checked through the phone and I don't see an option to shut off saving email addresses. To make matter worse these addresses cannot be deleted either.