iPhone can't verify server identity
Cannot Verify Server Identity
The identity of "google.com" cannot be verified
[Edited by Moderator]
What happens when you click the "Details" on the error popup message (See the Pic below)
Best would be to Delete and Add the email account that is creating the issue.
You may also try changing SSL settings
Settings > Password & Accounts > [Select the Mail account} > [Tape on Account with the email ID} > Advanced > SSL Settings > [Disable]
Click <Account (in Blue)
Click on DONE (in Blue) when exiting in the previous screen