From time to time, Apple's native email client will be erroneously blocked by Microsoft's email servers. The problem seems to be sporadic.
When this happens, you will see this message:
The solution to this issue is to use the Outlook iOS app. Using the Outlook app will also benefit you by more effectively separating your work email from your personal email.
Instructions to setup your company email in Outlook for iOS are located here:
Email on your Phone [iPhone Outlook App]