When Office 365 groups are created, the name provided is used for mailNickname, as well as for the first portion of the SMTP Address. Prior to this change, Office 365 group creation has not been enforcing that the mailNickname be unique across Office 365 groups. If multiple Office 365 groups contain the same mailNickname, you can encounter collisions when these groups are sync’d to on-premises via AAD Connect. In Azure Active Directory, an enforcement will be placed on the mailNickname property so that it will be unique across Office 365 groups. Previously created Office 365 groups that have duplicate mailNicknames will not be affected. If a user attempts to modify the mailNickname property through PowerShell or other means, the service will verify whether the new mailNickname being specified is unique. If not, the modification will be rejected. Additionally, a user can create an Office 365 group that has the same mailNickname as an Office 365 group that has been soft deleted. If a user attempts to restore the soft deleted group, they will be prompted to change the mailNickname.