I am running the WHMCS to WP integration plugin.
I also have a premium module installed that integrates cPanel into the WHMCS.
When accessing the cPanel email feature while WHMCS is 'inside' the wordpress site, the module no longer works.
But when accessing the cpanelExtended module on the direct WHMCS install the module works like a charm.
The module is found here:
The address of the WP install with the WHMCS is http://webdatacompany.com/client-webportal/
The WHMCS install is https://portal.webdatacompany.com
For a temp access to troubleshoot, please direct message me for it.