From this tab, you can easily send a single e-mail to a specified recipient (not only the site users). For sending e-mails to multiple users, we recommend using the Mass e-mail interface at Site Manager (CMS Desk) -> Administration -> Users -> Mass e-mail instead.
More resources can be found at:
Page url: http://devnet.kentico.com/docs/5_5r2/contexthelp/index.html?send_email.htm