Creating e-Mail account for multiple users at one domain

[JO Spies]

Good day

We would like to integrate an option to have clients sign up for an e-Mail account with an existing domain that we have, much like gMail for example.

Basically the client selects the option to register an account and is directed to the checkout page.  There is no need to have domain option enabled as the domain already exists.

Now they have to be able to type in their desired account@domain.co.za that we have available accompanied by a password.  The system should be able to show if that e-Mail account has already been taken.

The closest we have come to this is by adding an additional field in WHMCS, but client will only be able to insert desired e-Mail account and not have it check against the database nor insert a password.

All this will have to be done manually on our side.  This means no privacy for the client and extra admin....

Is there a 3rd party plugin that can help achieve this?

Any help would be greatly appreciated.

Regards,

JO