Solution 1:-

This error might Microsoft related with your mobile number. In your web browser, sign in your account and go to Settings. Then disable any number you have listed. Also disable 2 step security.

Solution 2:-

Turn off SSL.

Go to Settings  > Mail, Contacts, Calendars > Exchange account > Account. Then turn off “Use SSL”.

Solution 3:-

Rest network settings.

Settings ->General ->reset -> Reset Network Settings.

Solution 4:-

Sign in to Outlook Web App. Go to Options -> Phone -> Mobile Phones and delete your registered device.

Other solutions:-

  • Delete the exchange account. Reset the wireless network. The re-add the account.
  • Turn off WiFi. Try to add your exchange account using cellular connection.