One of the biggest drivers of technical support volumes for Office 365 is Outlook connectivity issues. To help assist, Microsoft has released a Guided Walk Through (GWT) for troubleshooting Outlook Connectivity issues in Office 365. You can access it at https://aka.ms/outlookconnectivity.
The purpose of this walk through is to assist in resolving these complex issues by focusing on the scoping and steps used to isolate and resolve problems. Therefore the walk through starts by focusing on commonly encountered symptoms related to Outlook connectivity.
Considering there might not be a single solution, but a combination of factors contributing to the problem, following the walk through will allow you to isolate and remedy the most common causes of Outlook connectivity issues to Office 365.
This walk through is not meant to replace all of the data that helps you understand Outlook connectivity issues, but rather to quickly give you the steps you need to help find the solution.
For more information, see Fix email connection problems