Cannot verify server identity

What To Do When Your iPhone “Cannot Verify Server Identity”

• Restart Your iPhone. If you receive an alert saying that your iPhone “cannot verify server identity”, the first thing to do is restart your iPhone. ...
• Close The Mail App, Then Reopen It. ...
• Delete Your Email Account, Then Add The Email Account Again. ...

Try a Reset All Settings (No Data is Lost)

This resets all your personalized features back to factory defaults

Go to Settings > General > Reset > Reset All Settings

• What this effectively does is to reset some of your Settings to factory default. It’s not a complete removal of settings. It removes all of your preferences for Wi-Fi, Bluetooth, Do No Disturb, Notifications, General, Sounds, Brightness & Wallpaper and Privacy.
• Resetting all settings also reverts all your personalized and customized features like wallpapers and accessibility settings back to factory defaults.
• Performing this action does not impact any of your personal device data, including photos, videos, notes, messages, reminders, emails, calendars, texts, documents, and other files