Back in 2014, Microsoft bought Acompli, an email client available to iOS and Android back then. Later they rebranded it as Outlook. One of the feature was Focused Inbox . It uses an algorithm which detects frequent contacts and push their emails above other emails kinda like how facebook shows content on newsfeed. While the feature had not been available to Windows 10 and Windows 10 Mobile but seems like Windows 10 version is getting it soon as its already in limited testing.
SOURCE – WC