Option to enforce client area e-mail address to be on a domain not hosted by that client
There are several problems that arise when a new client transfers their webhosting to a host (so already has example.com hosted elsewhere), signs up for a package to host example.com with the new host (intending to change their NS to point to this new host), and uses their personal e-mail address firstname.lastname@example.org for their client area account.
The longer term problems are that if the client were to be suspended, or if the server they are assigned were to go down, the host would have no way to contact them.
But the bigger problem is when the server hosting the WHMCS installation is in the same DNS cluster as the final hosting server, because then WHMCS will route the "new package details" e-mails to their new hosting package, even though the client hasn't yet got any way to retrieve those e-mails.
This should be optional, but it would help to have some kind of way to block this. Perhaps the option would disallow someone from creating a client area account using the same e-mail domain as they have in their cart, and/or prevent an existing client from ordering a new domain if the domain is the one used for their client area e-mail.
I'm not quite sure what the best way would be to do this, so I'm hoping others might pipe in with their thoughts.