proxyAddresses Field in Azure AD

Question

Will the proxyAddresses list in Azure AD always contain the value from the userPrincipalName field & the value from the mail field? I took a look at this documentation: https://learn.microsoft.com/en-us/troubleshoot/azure/active-directory/proxyaddresses-attribute-populate and this does seem to be the case, but I want to confirm. Basically, what I am trying to see is that if userPrincipalName has a value of x and mail has a value of y, then will proxyAddresses always contain both x and y? Of course with the values in the proxyAddresses list being pre-pended with "SMTP:" or "smtp:"

Answer 1

Office 365/Exchange Online enforces a policy that requires at least one of the aliases contained within proxyAddresses to match the UPN. This applies regardless of whether you are syncing the user object from on-premises AD or creating it directly in the cloud. It's not applied when the tenant is not using Exchange Online however (or the object is not licensed for Exchange).
And, changing the mail attribute results in adding the supplied value as the primarySMTPaddress, while preserving the old value as a secondary alias. However, you can provision objects initially with different values for mail and primarySMTPaddress, including having one of them being null.
So to answer the question, no, not always. But for most Exchange scenarios, yes.