Welcome to the Apple Support Communities. I see from your post that any app that accesses email freezes up when you try to type in the "To" field. I know it's important to be able to send emails without that kind of issue happening every time you try to address the email. I am happy to help.
If you haven't already done so, please restart your phone:
Restart your iPhone, iPad, or iPod touch
Next, remove the mail app, and then reinstall it. Details on both of those steps can be found here:
Delete built-in Apple apps on your iOS 11 device or Apple Watch
To reenable your mail accounts after you reinstall the mail app, go to Settings > Accounts & Passwords and enable the "Mail" slider for each account.
Let us know if you're still having the same issues after those steps.
Have a great day!