If you are planning to deploy the HCL Connections Mail Plug-in, take note of the KB0092821 knowledge base article. This is a mandatory step that must be done in HCL Connections 8 CR1 and newer environments.
If the steps described in KB0092821 article are not followed, you will get the following error message in the browser console:
Error: Unable to load https://<mailserver_hostname> status: 403
We are working on adding this step to the official documentation of HCL Connections Mail Plug-in.
Thoughts about HCL Connections, Domino and Sametime 