Sometimes Mail Server limit the amount of apps that can connect to them.
For example, Office 365 requires clearing some devices via Web Settings after reaching 10 connected devices limit.
Please check if you have an option to check number of connected devices via your Web console or Server Administrator.
Here are instructions for Office 365 and other similar accounts:
After you reach the maximum, you can’t create additional partnerships until you delete existing ones from your account.
Do the following:
1. Sign in to Outlook Web App
2. Click Options > Phone > Mobile Phones
3. Delete any unused/irrelevant/old/unneeded partnerships.
If these actions don't solve the problem - please let us know.