Excluding specific email address when sharing email across organization

Excluding specific email address when sharing email across organization

Hi There.

Please could you let us know if there is a way of sharing email across organization but excluding one specific email address (e.g. your private gmail). 

Our approach: We thought about excluding gmail domain in general, but this useless as all of our customers having gmail addresses would be filtered out.

It would be awesome if you could help or at least give a hint here.

Thank you in advance and best regards
Justus