When sending email (Outlook app or online) TO and email alias it substitutes the users primary email
So, 2 users in the same Microsoft 365 network: One has a primary email of Email address removed and the other has a primary of Email address removed with an alias of Email address removed. When sender sends an email to a third party and CCs Email address removed, the email is sent to Email address removed. The third party is naturally confused as to why an internal CC seems to be going to a different domain. That is, it appears that ABC is telling XYZ private information for no good reason - sender and receiver know this is all ok, but third-party doesn't How do we stop this?