This plugin adds emails to a mailing queue instead of sending immediately. This speeds up sending forms for the website visitor and lowers the server load.
Emails are stored in database which are deleted after emails are sent.
You can send all outgoing emails via an SMTP server or (the WordPress standard) PHP function mail, and either use wp_cron or a cronjob (if your server/hoster supports this) to process the queue.
Plugin requires PHP 5.4 or above.