Send test mail for smtp fails
From Joomla! Documentation
Websites updating to Joomla 3.9.23 trying to send a test mail using smtp auth may fail.
This pertains only to Joomla! version(s): 3.9.23
What is the cause
Due to a security update we no longer show passwords on the global configuration screen. The send test mail function uses the password from the current configuration screen which is empty after saving the configuration.
How to fix
Update to Joomla 3.9.24. In the meantime you have to entering the SMTP Auth password before sending a test mail. As alternative you can apply the Pull Request #31515 manually.