How to debug SMTP mail in Joomla 3
From Joomla! Documentation
Go to Extensions - Plugins and configure the plugin 'System - Debug' with the following settings:
- Allowed Groups: Super Users
- Log Priorities: All
- Log Categories: mail
- Log Almost Everything
Go to Global Configuration - System and turn on the option 'Debug System'. Then go to the Server tab and Mail Settings. Click the button 'Send Test Mail'. If the send function reports an error, you will find the details of the SMTP connection requests from Joomla and responses from the mail server, including the error codes, from the log file, usually administrator/logs/everything.php. If the website was updated from Joomla 2.5, the log file is logs/everything.php at the top level of the Joomla folder.