Yep... any email that's not pushed by the email server... like iCloud, and some Outlook based email services is fetched, and the shortest fetch duration is 15 minutes... every time you open the stock iOS Mail App - all accounts are refreshed and new mail is fetched.
I have all my mail set up on Manual anyway so to me it makes no difference. I just check it when I check it and for those super important ones, Those people know to shoot me an iMessage to check my mail - Important! Works good for me this way.
It can be frustrating if you are waiting on an email response on your iDevice. Yes going out of the email app and back in again will refresh and fetch.... Fetch always makes me laugh...
You can also force the refresh while on your email by swiping from the top down and you will see notice at the bottom of the screen that it is checking again for new mail.