Hi @Dan , you can configure Azure AD to let users sign in with their email as an alternate login ID. This feature enables sign-in with ProxyAddresses, in addition to UPN, for cloud-authenticated Azure AD users.
However, you need to ensure that the domain in the email address is a verified custom domain in Azure AD. If the UPN suffix doesn't match a verified domain in Azure AD, then Azure AD replaces the UPN suffix with the default onmicrosoft.com domain.
Please let me know if you have any questions and I can help you further.
If this answer helps you please mark "Accept Answer" so other users can reference it.
Thank you,
James