Forgive the rambling of one who is only guessing.
An email address is like the old fashioned poste restante, a store of email at the ISP server.
The member's computer looks while online for any mail and downloads the fact that there is mail waiting. At that point it can be downloaded or deleted.
Some vital line in the registry allowed access to the old ISP at the same time as it was connected with the new.
I'd be surprised if it happens again now you have cleaned the registry.