Our user's specify username@domain.com to check virtual domains. However, some old clients (old Eudora, and sadly, some of our customers still use it) do not ask you to specify a POP3 server. You simply input your e-mail address and it uses the domain from it as the server, but only sends the username. For these customers we specify username@domain.com%mailserver.domain.com as multiple @'s on one line is an error....