A little notice on a potential helpful feature which was introduced to Outlook at some point, but I wasn’t aware of before (or it’s just new). At least the option is available in Outlook v1905 build 11629.20008 C2R; it might also be available in standalone.
Many people are familiar with the Outlook Connection Status window, which you can summon by right-clicking the Outlook icon in the system tray while holding CTRL. This will show a dialog containing the connections Outlook is managing for every configured account, together with valuable information like endpoint, response times, etc.
One of the columns, Req/Fail, is showing the number of Requests and Failed requests. To check the headers of the last failing response for a particular connection, double-click the Req/Fail number. This will open up a popup window similar to this one:
Apart from essentials like the http result code, it will show which front-end and back-end servers processed the request. This might help to quickly determine if clients are connecting to unfavorable public endpoints, or when failed requests are coming from specific in case of Exchange on-premises. Of course, this information can also be retrieved using additional tools like Fiddler, but with this shortcut you don’t need to install additional software, as well as that you can ask end users to open up this window and send you the information.
Again, another little gem which might come in handy when troubleshooting.