TLS 1.3 is the fourth version of the commonly used protocol for secure communication on the internet. Unfortunately, not all apps and systems can work with it correctly now, such is the case with Windows 10.

You can view which Windows versions can work with TLS 1.3 on this Microsoft page: TLS protocol version support

Gmail OAuth login on Windows 10
When TLS 1.3 is enabled, this can cause issues with the Google accounts OAuth - the authentication window will show up, but the setup can never be finished, the OAuth window will keep reloading and the eM Client app will not be able to connect to the Gmail server to synchronize your data.
The solution is to run the DisableTLS1.3.reg file attached to this article, start eM Client, and try the Gmail account setup again.

A similar issue can affect Kerio accounts when using Windows 10 as well. The solution is also to disable TLS 1.3 until Microsoft provides the required update to Windows 10.

This solution can also work if you encounter this error when sending or receiving an email:

"Cannot determine the frame size or a corrupted frame was received."