Some users have reported "can not verify server identity".
This is caused by the expiration of a Certificate Authority on 9/29/21.
This problem should NOT affect iPhones and iPads with iOS >= 10.
But it does.
While a solution for this problem is being investigated further you can turn
off SSL (which requires the Certificate Authority) on your iPhone Mail App.

Disable SSL on Iphone Mail App
   1.  Click on Settings
   2.  Click on Mail, Contacts and Calendars
   3.  Under Accounts select your Email Account
   4.  Click on your Account again
   5.  Scroll to the bottom of the account screen and click on Advanced
   6.  Scroll to the bottom and under Incoming Settings disable the option Use SSL
   7.  Make sure Authentication is set to Password
   8.  Click on Account located in upper left hand corner
   9.  Under Outgoing Mail Server click on the SMTP server
   10. Click on the Primary Server
   11. Under Outgoing Mail Server Use SSL turn that off
   12. Make sure Authentication is set to password and that you do have your username and password in respective fields
   13. Click Done
   14. Click Account in the upper left hand corner
   15. Click Done again
   16. Now you click to go back to your home screen