The WHMCS need to have an option for Web Designers and Developers manage their clients with a single e-mail account and password.
The developer X has 5 registered clients in WHMCS (5 accounts),
it can add to each of these accounts an email master.
So the customer has to charge your name and developer access to the profile in order to work.
There is the option "clients contacts", but it is not good, a professional who has 50 clients need 50 email accounts? nonsense.