I am using SharePoint Online. I am trying to invite one Internal user to my SharePoint site but SharePoint is not recognizing that user. If I try to find that user using his email address then SharePoint is giving me message that
firstname.lastname@example.org is outside of your organisation.
This is an old user. when I go to 'SharePoint Admin Center' and search this user in 'User Profiles' then I can see this user. That means user is valid and successfully synced with SharePoint.
Any idea why SharePoint cannot find this user when I am trying to share my site with him?