Cannot Verify Server Identity ,

Hello Miomiomio,

Welcome to Apple Support Communities! It sounds like you need help with email issues. We completely understand why you'd reach out, and are happy to help.

This dialog indicates that there may be a certificate issue with your e-mail provider's mail server, or there may have been a change to their systems at some point recently. As a first step, I'd try removing the account and setting it back up via the steps outlined below.

1. On your computer, sign in to your email provider’s website to verify that all of your email is there or check if your email is saved somewhere other than your iOS device.
2. On your iOS device, tap Settings > Mail, Contacts, Calendars.
3. Tap the email account you want to remove.
4. Tap Delete Account.
5. Add your account again.

If, at that point, you're still getting the message regarding the server's identity, the next step would be to contact your email provider.

• Contact your email provider or check their status webpage to see if they’re experiencing a service outage.

• Ask your email provider or system administrator if you have any security features or restrictions, like two-step verification, turned on for your email account. You may need a special password or need to request authorization from your email provider to send and receive email on your device.

• Check your email account settings with your email provider or system administrator to make sure that they're correct.

Get help with Mail on your iPhone, iPad, and iPod touch

Please have a look and let us know if this helps! 

Cheers!