If you have the type of email account that benefits from "Push", like with iCloud or those hosted via an MS Exchange server, watch this video for help with your iOS 8 Mail app.
Or follow these steps
- Choose "Settings" from your iOS device home screen.
- Select "Mail, Contacts, Calendars".
- Tap into any email account listed and check that (if available) the "Mail Days to Sync" is set to what your email account provider recommends or to the time-frame that best meets your needs.
- Go back one screen to "Mail, Contacts, Calendars" and look for "Fetch New Data", to tap into that section.
- Select the email account and see that your Sane folders are check-marked for push, so they stay in sync with your mail server.
Note: While setting push for Sane folders you can also set push for folders like Trash or Spam, just in case you need to see the current state of mail there too. Other than for Sane folders and maybe Trash, set for push of other folders based on your own personal preference.