Our corporate IT manages our O365 so I do not have a way to make changes. Our UPN for login to O365 is employeeID@domainname.com. When someone shares something with one of our users via email address, the user in our tenant goes to login to the SharePoint resource and is presented with the message "You're trying to accept your invitation with employeeID@domainname.com because that is what we use to login. Entering the email address presents the message "the username may be incorrect...." Do we have to provide users with employeeID@domainname.com instead of email address. Mail to employeeID@domainname.com works.