You have reached your device limit

There is a ten device limit for using email on mobile devices. If you regularly change phones/tablets etc you may well hit this limit which will then stop you from being able to set up your email on the next mobile device. 

You will also receive an email about this along these lines

From: "Microsoft Outlook"
Date: Sat, Jan 14, 2017 at 4:45 PM +0000
Subject: Error with your new mobile phone partnership

You have 10 phone partnerships out of the maximum allowed 10 partnerships. After you reach the maximum, you can't create additional partnerships until you delete existing ones from your account. To do so, sign in to Outlook Web App, click Options > Phone > Mobile Phones, and delete any unused partnerships.

Basic instructions to fix are included on this email but here's the step by step

Sign into webmail
Click options on the top right and choose "see all options" from the drop down
Click phone on the left hand side
You will then see a list of devices you have set up your email on, delete the un-needed devices by selecting them one by one and clicking the red "x" above to remove them