Hi AaronGhadiyali,
Thanks for posting your concern here. To set up Office 365 Exchange account in the built-in Android mail app, please make sure you follow the steps in the link below carefully.
https://support.office.com/en-us/article/Set-up-email-in-Android-email-app-71147974-7aca-491b-978a-ab15e360434c#ID0EAADAAA=Gmail_app
In addition, please try to enter "outlook.office365.com" in the server setting and view whether you can add your account successfully.
Moreover, to check whether the issue is related to the built-in mail app, if condition allows, please try to add other account to the build-in android mail app and let me know the outcome.
If the issue persists, please capture a screenshot about the configuration of your account in the built-in mail app and let me know the version of mail app. By the way, we recommend you use Outlook for Android app for better using
experience, which has more features and technique support.
Best Regards,
Betty