Howdy Boxahpowah,
I understand that the Mail app is quitting unexpectedly on your iPhone. Since restarting the iPhone (which is always a great first troubleshooting step to take, by the way!) didn't help, the next step forward would be to force close all open apps and force restart your iPhone (by holding the Home and Sleep/Wake buttons until you see the white Apple).
Force an app to close in iOS
http://support.apple.com/en-us/HT5137
Turn your iOS device off and on (restart) and reset
http://support.apple.com/en-us/HT1430
If Mail continues to quit unexpectedly, the next step going forward from there would be to remove and readd your email accounts in Settings > Mail, Contacts, Calendars.
- Tap Settings > Mail, Contacts, Calendars.
- Tap the email account.
- Tap Delete Account.
- Add your account again.
Get help with Mail on iPhone, iPad, and iPod touch
https://support.apple.com/en-us/HT201419
Sincerely