@lubos, what does manager default to? I think that is the pertinent question. If you haven’t set the default to v1.2 or you are not using the system default, then manager doesn’t support servers that reject v1.0 since dotNet4.6.61 uses the wrong one. It has been extensively reported on.
Since lubos has indicated Manager supports the required TLS, and at any rate you’re on Net4.8, there is obviously an issue with the credentials.
You need to work out at what layer it’s happening, since the mail server doesn’t usually indicate TLS errors you’re going to have to look at packet captures. Having said that, you are getting explicitly not authenticated for anonymous mail Which to me does sound like a login issue. Perhaps that may be the same error with TLS, I’m not sure.
Grab a packet capture and see what it says.