net core 5.0 An error occurred

net core 3.1 is normal
Works on my machine, with .NET 5. Looking at the stacktrace, it's not an issue via FluentEmail, but with the SmtpClient and the mailserver.
Then why does it work normally under net core 3.1? I have not changed anything except net 5.0.
Are you sure you are using the same smtp server and you are connecting to it from the same network? Might be a relay issue, were you can call it from corporate network, but not from home.
I am sure I am using the smtp server, and the code and make any changes, except from net core3.1 to net 5.0
@qcjxberin the tests seem to pass on net 5. so can you share a minimal repro pull request in the form of a failing unit test?