How to debug SMTP mail in Joomla 3
From Joomla! Documentation
Go to Administrator → 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 Administrator → System → Global Configuration → System tab and set Debug System to Yes.
Then go to Administrator → System → Global Configuration → Server tab → 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, in 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.