Manual:$wgSMTP

Details
For using a direct (authenticated) SMTP server connection, you need to fill an array. Set to false (the default value) to use the built-in PHP mail function.

For example: See Mail::factory for a full description of these settings. (Scroll down to the  parameter.) Warning: This setting requires PEAR's Mail package to be installed (run "pear install MAIL" and "pear install Net_SMTP").