Microsoft's recommended best practices are to match UPN to primary SMTP address. This article addresses the small percentage of customers that cannot remediate UPN's to match.
Note: A UPN is an Internet-style login name for a user based on the Internet standard RFC 822. The UPN is shorter than a distinguished name and easier to remember. By convention, this should map to the user's email name. The point of the UPN is to consolidate the email and logon namespaces so that the user only needs to remember a single name.
Configure the Azure AD multifactor authentication registration policy
Azure Active Directory (Azure AD) Identity Protection helps you manage the roll-out of Azure AD multifactor authentication (MFA) registration by configuring a Conditional Access policy to require MFA registration no matter what modern authentication app you're signing in to.
Reference:
https://docs.microsoft.com/en-us/windows/win32/ad/naming-properties#userprincipalname