SMTP plugin - why "Token has been expired or revoked."

My client’s SMTP plugin stopped working. On the dashboard I got a message (see below). I had to disconnect the email and reconnect it, just with email and password. Settings stayed the same. Does anyone know why this happens?

You can check the plugin’s support forum and search for that and see if anyone answered it there.

Thanks. I tried that. It’s not a clear answer. Many possible causes. It could be because my client changed his email password but my client was pretty sure he changed it after the plugin stopped working.