Some time ago we had identified that if people (Mail user) or user account with the email address entered in the email field of your Active Directory were present in your tenant’s Azure Active Directory environment, then these people could not be invited as guests in Teams. A priori I would say that this is confirmed. The MAIL user functionality in Office 365 can no longer be used if you want these people to have the opportunity to be invited into team. Because if the user can be resolved then the user will be considered as internal. If you are in hybrid mode here is the action plan I propose
All User accounts with mail extension (Mail User) are synchronized as such in 0365
- This synchronization makes them appear in the address book of Exchange Online and Exchange On Prem. Teams cannot see them as an external person because they are considered internal.
All user accounts with the email address (in the email field) entered are synchronized as such in 0365
- This synchronization does not show them in Exchange Online or Exchange on Premise. Teams cannot see them as an external person because they are considered internal.
Recommended solution for users with Mail User Extension
– Inventories of their membership in Exchange distribution lists
– Convert all users with Mail Extension as a Single Active Directory user
– Creation of an email contact with their email address in the 0365 or Onprem environment with synchronization to Azure AD
– Repositioning in distribution lists
Recommended solution for users whose email address is entered in the Active Directory email field
– Stop the synchronization of this type of accounts in the 0365 environments