Is there a way to use the proxy using the new IMAP and POP.AccessAsApp Permissions in Office 365?
In this case the administrator could handle the authorization and set the permissions for the account in Office 365 and the handling of authorizations on the gateway would not be necessary.

Reference: https://docs.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth

Thank you for your answer -tom