1) shared mailboxes (that multiple people can manage)
2) sane auth[entication|orization]
3) retention of deleted emails
then there are only 2 players in the game: Microsoft and Google… and once you pick one of them, it makes sense to also buy into the rest of their respective ecosystems, since everything is well-integrated.
The choice then ends up coming down to whether your users prefer Google Docs/Sheets/Mail or Microsoft Word/Excel/Outlook…