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 mailto:Email address removed and the other has a primary of mailto:Email address removed with an alias of mailto:Email address removed. When sender sends an email to a third party and CCs mailto:Email address removed, the email is sent to mailto: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?