I’m currently involved in a project where is it requiered to team up with other companies and provide their employees access to my client SharePoint sites but nothing else. Why? because the business core is very restrictive and is not allowed to share documents or sites with external users and being internal users is more feasible to control which actions they can do.
The main point here is to create Active Directory account synchronized with O365, but thise users have not licenses associated to them, so is it possible to be add to SharePoint using their Active Directory account, but not to access to other resources
As you can imagine, the main requirement is to not assign extra licenses to this users to receive emails, so we have to spin the wheel. So the main point here, is when we create the user in OnPrem, is it necessary to include the work email address.
Once this user has been synchronized in O365, we can check the SharePoint Online user profiles parameters, so we are able to see if this user has been synched correctly or not.
So we go to sharepoint admin center > user profiles > people > manage user profiles> input the user name which you want to change in the find profiles dialog box and click find.
Once we have checked that the information shown in the profile is the correct one, we can go on and test if its possible to send notifications to this external user using the mail previously configured.
That’s all, at first moment I thought that it would be more complicated to receive notifications without and Exchange Online license, but the only thing to take into account in this equation is to inform the user email address OnPrem or change the info of the user in the User Profile of SPO.