Hi there scubabob,
We understand you have an issue with the Mail app on your iPad Air closing unexpectedly. You've mentioned already restarting your device, which is a good first step. Since this app specifically is having the issue, we'd recommend you uninstall it (delete), restart your iPad Air, then reinstall the Mail app.
Delete apps on your iPhone, iPad, and iPod touch - Apple Support
1. Touch and hold the app.
2. Tap Remove App .
3. Tap Delete App, then tap Delete to confirm.
You will want to have your email account details on hand, since this will remove the configured account information. You will need to set up any mail accounts again.
Add an email account to your iPhone, iPad, or iPod touch - Apple Support
Let us know how it goes.
Best regards.