Workmail Failing on iPhone for One User Account Only

We have multiple users in our organization, all who use WorkMail on mobile devices, specifically iPhones. Every account works well except for one. I can see no difference in permissions or in how it was set up, the account simply fails to validate when configured automatically or manually. We have tried the same account on multiple phones, from IOS 11-14. We have tried all of the tricks in the setup guide including manually configuring with our server etc. Any ideas?