How to update the proxyAddresses of a Cloud-only Entra ID user

Even though the Graph API allows you to read the proxyAddresses property via GET, modifying it with PATCH often fails for cloud-only users without Exchange Online. This is because:

• The proxyAddresses attribute is tightly coupled with Exchange Online.
• Without a mailbox, the backend doesn’t expose full write access to that attribute, even via Graph

Option 1: Provision an Exchange Online License

Assign the user an Exchange Online license, which will provision a mailbox. Once that's done, you'll be able to manage proxyAddresses using Exchange Online PowerShell or Graph API. This gives you full control over mail-related attributes, but it does come with licensing costs and setup time.

Option 2: Recreate the User

Delete and recreate the user with the correct proxyAddresses values from the start. This ensures a clean configuration, but it can be risky, you might lose group memberships, app assignments, and audit history unless you carefully reassign everything.