"Cannot verify server identity" message from mail

According to Apple support, if you have more than one email account defined. 
As long as that entry exists in the outgoing email server list, iOS will not present the option to trust its certificate after the IOS 10.2.1 update.

Solution:

1. Go to settings > Email Accounts.
2. Delete all accounts that are not working under our hosting
3. Then add them back again