How to remove cannot verify server identity off iphone 11

Try the following, in order, until, hopefully, resolved:

1. Reboot your phone. Ref: Restart your iOS device
2. Close, then, reopen the Mail app.
3. Delete your eMail account, and then, re-add it back.
4. Reset all of the iPhone's settings. Ref: Settings > General > Reset > Reset All Settings

Tesserax offers excellent advice. But I have found just wait it out and it goes away on its own.

That would be too long. Have you tried the steps I provided you yet?

You're welcome and I'm glad you got it work out.

If this has been going on for a week, you need to do what Tessserax suggests. For me, when this happens it's usually just a very short time frame, like an hour or two.