Update troubleshooting.md

src/essentials-for-yootheme-pro/troubleshooting.md

@@ -11,3 +11,7 @@ If you see this error the Essentials files integrity check has detected that som
 ## AbstractRule Class Not Found
 
 A fatal error displaying `Class "ZOOlanders\YOOessentials\Access\AbstractRule" not found` most likely is related to a 3rd party plugin trying to access this class before Essentials has been able to declare it. The solution is simple, set the 3rd party plugin order with lower priority than Essentials.
+
+## Where to set SMTP credentials
+
+In Joomla you can do so in the `Global Configuration` while for WordPress you need to install a SMTP plugin, like [WP Mail SMTP](https://wpmailsmtp.com), and do so in it configuration.