Hi Ruxpin60,
If you are having issues with your iPhone's Mail application not launching properly, you may want to try some things to troubleshoot.
First, quit all running applications and test again -
Force an app to close in iOS
Next, I would try restarting and if needed resetting the iPhone -
Restart or reset your iPhone, iPad, or iPod touch
If the issue is still present, you may want to restore the iPhone as a new device -
How to erase your iOS device and then set it up as a new device or restore it from backups
Thanks for using Apple Support Communities.
Best,
Brett L