Seed email accounts are one method. I know ReturnPath had hundreds (thousands?) of mail accounts at various ISPs and that would be included on mass mailings and

Seed email accounts are one method. I know ReturnPath had hundreds (thousands?) of mail accounts at various ISPs and that would be included on mass mailings and checked. You know it's supposed to show up because you sent it -- does it? If not you might be getting marked as spam.