Duo SSO can be used to protect any applications that let you set up SAML authentication. I don’t believe you can do this with Outlook Web Access but would be able to with Microsoft 365 web hosted mail using [Duo Single Sign-On for Microsoft 365 | Duo Security].
Duo SSO can’t natively work with RDP but we do have currently in Public Preview Duo Network Gateway support for RDP. The Duo Network Gateway is a customer-hosted server that can proxy connections between external users and internal services and can use Duo SSO as its SAML IdP.
In this scenario instead of using Duo Authentication for Windows Logon and RDP you’d set up the Duo Network Gateway to interface with your RDP servers and would not need to directly expose them to the public internet or require users to VPN to access them. When they go to authenticate to their RDP servers using Remote Desktop they’d have a browser window pop up to complete Duo SSO authentication before then being connected to the RDP server to complete normal authentication.