Just because you’ve moved to Exchange Online doesn’t mean you no longer have access to run tests on your (or your customers) Exchange environment.
In case you haven’t seen it – Microsoft have a free online testing service called the Exchange Remote Connectivity Analyzer (https://www.testexchangeconnectivity.com).
If you’re encountering problems with connecting from any device (ie. Outlook on PC, Outlook on Mac, mobile devices, etc.) or simply having difficulties with mail flow – this is the tool for you.
Simply select the test you want to run, type in the relevant details (ie. email address, username, password, etc.) and away it goes.
Apart from testing if services work, it will provide you with a heap of deep level configuration information in the event of a failure.
A great site to keep in your monitoring & testing arsenal.
P.S. Also works with on-premise Exchange servers! (remember those?)