Hey chrisjfinlay,
We've reviewed your post and it appears that you've set a default mail app, but when trying to start an email outside of that app, it will open the Mail app only and not the chosen email app.
To confirm, you set the default mail app by following the directions below from this article, correct? Change the default web browser or email app on your iPhone, iPad, or iPod touch
- Go to Settings and scroll until you find the third-party app.
- Tap the app, then tap Default Mail App.
- Select an email app to set it as the default. A checkmark should appear next to the email app to confirm it's the default.
If you want iOS Mail as your default email app, but you previously deleted it, you need to restore it. Go to the App Store and search for Mail. Then tap the Restore button 
. Wait for the app to restore, then open it from your Home Screen. Follow the steps above to set iOS Mail as your default email app.
If you don't see the option to set a third-party email app as the default, the developer might not support it. Contact the app developer to learn more."
Does this issue continue if you switch the default app, restart your iPhone, and then switch it back? Restart your iPhone
Also, does it help to delete and reinstall the two apps? Before doing that step, we recommend signing into your email accounts via webmail to ensure all your emails are showing there before taking that step or that the emails are saved somewhere else than your iPhone. These two articles can be helpful for that step: Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch and Redownload apps and games from Apple
We look forward to your response. Take care!
