1. Outlook will sync different email addresses, and allow you to filter your view of them by each different email, viewing them all at once, or even by importance.
2. You can reschedule an email to reappear in your inbox at a different time as a reminder to yourself.
3. iCloud, Google Drive, Dropbox, Box account, and of course, Outlook can all link to your email, allowing you to add attachments from these services.
4. There is a built in customizable calendar that draws information from all linked accounts.
5. There is a contacts menu of most recently contacted people, from which you can filter your emails by person.

All information taken from "5 reasons why you should ditch Apple Mail and go with Outlook for iPhone instead" article on pocket-lint.com.