We often get asked for support about SMTP connections, sadly there is not much we can do about it. All the information you need will be provided from your email provider.
With the newest release (from 15th of April 2022) we have build in a easy way to figure out what is going wrong when you setup a SMTP or OAuth2 connection.
Simply open following file: operator/settings.php and find:
// $mail->SMTPDebug = SMTP::DEBUG_SERVER;
$mail->SMTPDebug = SMTP::DEBUG_SERVER;
Run the test connection again and you will receive a detailed error message what is going wrong, most of the time it is the wrong email host or username/password.
As soon you get a success message and receive your test email you can undo the changes in the settings.php file.