While configuring email on iPhone, we can use IMAP settings. We can use IMAP and SMTP server address as mail.domainname.com. However, if the SSL certificate installed to the domain name, then only we can enable 'use SSL' for incoming and outgoing server. When the SSL certificate is not installed to domain name and we enable SSL in settings, it gives error like 'Cannot Verify Server Identity'.
If you still have issue, we can follow below steps to fix it.
1. Restart you iPhone
2. Close the Mail App and re-open it.
To close out of the Mail app, double-click the Home button, which will open up the App Switcher. Use a finger to swipe up on the Mail app until it no longer appears in the App Switcher.
3. Delete Your Email Account, Then Add The Email Account Again
To delete an email account (it will not delete the actual email account on server) on your iPhone, open the Settings app and tap Accounts & Passwords. Then, under Accounts, look for the email account you want to delete and tap it. Finally, tap the red Delete Account button at the bottom of the screen, then tap Delete Account again when the confirmation alert appears on your iPhone’s display.
Next, reenter the account information by opening the Settings app and tapping Accounts & Passwords -> Add Account. Then, select your mail service and enter the information.
4. Reset All Settings
To reset all settings, open up the Settings app, then tap General -> Reset -> Reset All Settings. If you have a passcode or a Restrictions passcode, you’ll be prompted to enter them. Once you do, tap Reset All Settings when the confirmation alert appears at the bottom of your iPhone’s display.