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:

'IDHost' is a MediaWiki-specific setting used to build the Message-ID email header (see RFC 2822, sec 3.6.4 for more information on a properly formatted Message-ID). For the remaining settings, see Mail::factory for a more complete description. (Scroll down to the  parameter.)

This setting requires PEAR's Mail package to be installed (run "pear install MAIL Net_SMTP", and you can run "pear list" to confirm).