Use the Local File Editor plugin, and edit local/config/config.inc.php. I have my own sendmail server for my home apps to use, so my config is straight forward. I just need to give it the IP address. This is configured to send everything through gmail as a smarthost. I could tell each app to send directly to gmail, but I'd rather keep the mail configuration centralized.

Check with you email provider about how to configure apps to use it.

// smtp configuration (work if fsockopen function is allowed for smtp port)
// smtp_host: smtp server host
//  if null, regular mail function is used
//   format: hoststring[:port]
//   exemple: smtp.pwg.net:21
// smtp_user/smtp_password: user & password for smtp authentication
$conf['smtp_host'] = '192.168.10.15';
$conf['smtp_user'] = '';
$conf['smtp_password'] = '';

// 'ssl' or 'tls'
$conf['smtp_secure'] = null;