It's possible to create alternative UPN suffixes and assign these to users in the domain.

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Quizplus · Accepted Answer

It is possible to create alternative User Principal Name (UPN) suffixes and assign them to users in the domain. This can be useful in scenarios where users need to log in using a domain name that is different from the default domain name. For example, users might need to log in using an email address instead of their username, or they might need to log in using a shorter, easier-to-remember domain name. By creating alternative UPN suffixes, administrators can give users more flexibility in how they log in to their domain.

It's Possible to Create Alternative UPN Suffixes and Assign These