I don't think we can keep track of email hosts and personal domains. Along with the common ones, there are hundreds of smaller email hosts.
Another problem would be that we have ultimately no control of what is submitted. We can modify the WHMCS plugin to ignore gmail and others, but we receive encrypted data at the end. Anyone with a little malicious intent can manually submit an email host domain that we are not aware of. We may end up flagging many innocent people.
The spirit behind FraudRecord is that we do not accept generalized data. A search of "John" ends up empty, since we require precise encrypted definition of individuals. I'm not sure how email domain fits into this picture.
"Domain Name or Hostname" is an existing field in the WHMCS module, which can be manually populated during a report or query. I think utilizing that is the best practice. It simply sends us data under "domain" variable, and can be adapted to any other custom API module. And that's pretty much specific to a single client.
