Sailthru can send campaign emails to large numbers of people rapidly. This is appropriate for newsletters, massive notifications, breaking news, and so on.
Every campaign email is sent to a mailing list. When first getting started, you will want to upload your initial lists. You can do this via the Manage Lists menu item in the interface.
You will receive an email when your list has finished importing; depending on the size of the list, it could take a few minutes.
Of course, going forward you will not want to manually maintain your mailing lists. You can easily subscribe and unsubscribe emails to your list using the email API call.
$sailthru->setEmail('example@example.com', null, array('subscribe_list' => 1));
For more on this topic see integrating mailing lists.
Now that you have a list to mail to, you can set up a campaign email via the Campaigns tab in the interface.
If you want to automate this process, you can do so with the blast api call.