We are facing authentication issues in SharePoint 365 when other companies tenant shares their resources with our tenant users. Currently our AD users UPN & email are not matching. The issue we have is when other companies share something in SharePoint to our company employees, they receive an email with the invitation link to the shared resource, when our users click on the link, they are authenticated first by our O365 tenant and then go to the shared link. The problem here is that SharePoint tries to match the email used in the invitation with the UPN of our users, not the email and because our UPN des not match with the email then they are rejected.
Is there any other option rather than changing the UPN of all users? anything we can do from our company side.. like in ADFS or etc.