I have an Azure AD (MS Entra - no on-prem) user account that does not have an Exchange licence, but has had an email address set up.

I want to use this "upn" (which is set as the email address too, as you would expect) as an ALIAS on another user account to receive incoming email.

However if I try to add the email address to the other user, I get an error saying another user (gives upn) is already using this email address. But it is not, as it is not licenced!

In Azure AD for the original user, I can see the "email" property does indeed still hold the "email address" I am trying to use for alias addressing. If I try to blank the "email" property field I get this error, and cannot save the user record:

Invalid value specified for property 'mail' of resource 'user'

I am guessing that somewhere (possibly in Exchange) there is part of a record left over, that needs to be removed.

Any ideas?