Could you explain in further detail as to the methodology? I am apparently having the same issue mentioned in the comment to which you replied, pertaining to Trust Certificates and the inability to use Mail, iCloud, and mainly any Apple function dedicated for iOS 10.2.1, which is the version I am currently running on my iPad Air 2. I typically prefer to use Gmail, however I have other email accounts I could use if necessary.
Just for further clarification, ever since upgrading along the way from iOS 9 to iOS 10.2.1, I have experienced so many glitche, such as my iCloud and iTunes account password is requested incessantly, even once input correctly, I receive error messages about not being able to access iCloud or iCloud Drive. This has not only become annoying, it has affected my Notes, iCloud, Mail, Safari and perhaps some other pertinent daily functions. Regarding Safari, my open Tabs consistently close without reason, along with all of my History, Reading List, Favorites and Bookmarks.
For instance, if I want to send an email from the Share button from Safari, I hit the Mail button, which asks me to set up email accounts, as if I have never set those up before. Just for my own humility, I will re-enter my email accounts again and again, some which come back with a message indicating that they are already registered with the System. Otherwise, I have tried using other email accounts, which have not been registered on my iPad. At which point, I go through the hassle of setting up the email account, Save the account, I receive a message that the account has been saved, however, when I go to Share, hit the Email button, it redirects me to the same screen to set up my Email accounts.
It sounds from your Discussion thread that you are referring to this type of matter, unless I am mistaken. If so, can you please offer up some more detailed advice, as I have spent hours on the phone with Apple Support, to no avail. And, the bottom line is, as much as I know about computers and networking, I wouldn't have any idea how to access, say, Google's Gmail Trusted Root Certificate and be able to email it to myself as an attachment.
As an aside, I do operate on a Windows 10 PC as well, in case this would need to be resolved using another computer source, other than my iPad Air. Anything you could do to assist would be greatly appreciated.
Regards,
Mark Halsey