Exchange On Premise and Exchange Online Authentication

Question

anyone can help me understand how to authenticate to exchange server for a server application on behalf of a user? My use case is to sync the mails of a user's mailbox and update the read state and deleted states with our server. Exchange online has rest API and Oauth to authenticate but that does not work with On Premise account? Is there a way I can authenticate for all the different deployments[exchange online and on premise] and access the Outlook mail?

We need to way to authenticate the user without trying to getting any credentials, so that our service can access mailbox without any need for prompt.

Answer 1

Seems your best bet would be to use impersonation, which would work in either EXO on on-prem. Your sever would require a service account in Exchange, then authenticate with the creds for that single account. Before synching mailbox, it would set the impersonation ID to the MB it was synching. Of course an Exchange admin would need to set up impersonation rights for your service account for every MB your service would examine, but that's easily done with a little PowerShell, and willingness on their part, of course.