The Send a test email button on the ACP/Email Settings page does not authenticate properly using SMTP. The reason is that the base64 encoded password sent to the remote server is actually ******** rather than the real password.

Email via other means (normal notifications or mass email) with same configuration works. It's just testing from the Email settings page.

_ref: https://www.phpbb.com/community/viewtopic.php?f=556&t=2397786&p=14586811_