-Users from certain Joomla groups.
-Users returned from SQL Query (for example, customers who purchased certain products in Virtuemart or all registrants of a specific events in Events Booking....).
-Users stored in a CSV/Excell file
Unlike other extensions which send emails as soon as the emails created, Mailer Pro store(queue) created emails in database and each time it only send a small number of emails (configurable - for example 50 emails each time). By doing that, you don't have to worry about mail server limitation anymore( Usually, Mail Server only allows sending about 150 emails per hours, and if you try to send more emails, these emails won't be delivered).
You can also use any content plugins in the email message send to your customers, for example, you can notify your site users about new articles, new products, new events..which are recently added to your site...
You can publish a system plugin comes with the extension (or setup cron job) to trigger emails stored in the system.