You may already know Fiddler, the web debugging tool which has been initially developed by a Microsoft support engineer years ago and now part of Telerik (but still free).

Well, the Exchange product team has been proud to announce the availability of a Fiddler extension dedicated to Exchange Online troubleshooting.

The Exchange Online extension provides you all the features available with Fiddler (session capture, session colouring…) but also much more dedicated to Exchange Online connectivity troubleshooting (from authentication to performance, or connectivity issues – not limited to Outlook client but also for Outlook on the Web).

The Exchange Online extension gives you additional tab available in the Fiddler interface with dedicated/specifics for Exchange Online connectivity like when an error has been detected it tells you what should have been expected (like for example the forwarding email format which should be xxx.mail.onmicrosoft.com or AD FS connectivity issues).

To use the extension you must first have the latest version of Fiddler available here https://aka.ms/FiddlerDownload and the .Net Framework 4.6 (or later) installed.

The extension is available from https://aka.ms/EXOFiddlerExtension

To get the complete details about it go to https://blogs.technet.microsoft.com/exchange/2018/12/11/introducing-the-exchange-online-fiddler-extension/