Hello,
the OAuth 2.0 for SMTP specifies, how the client sends the OAuth token to server. From this point - yes, Rebex Secure Mail supports OAuth 2.0 authentication for SMTP.
However, to receive the OAuth token is a little bit tricky. Typically, at first you have to register your app at the server's system, so the Yahoo, Outlook, etc. will know which app will request OAuth authentication, what services it will use, etc. Then you have to request an OAuth token for the user. And finally, you can use the received OAuth token and authenticate using your app. From this point - no, Rebex Secure Mail doesn't support OAuth 2.0, because you have to request the token "manually".
We wrote a blogpost about using OAuth with Gmail. You can follow it to add support for Yahoo, Outlook and Hotmail as well.
Update: We wrote another blog post that describes how to login with OAuth 2.0 to Office365 with Rebex Secure Mail, and another one that describes how to register application for with appropriate permissions in Azure.