E-mail Client

Is the Outlook client the only option connecting to the Exchange/Outlook server?

As of 5/01/2020 - yes Outlook (ActiveSync) is the only option for connecting to Office 365 e-mail, or using the web Client. 

If you are using a Windows or Mac you can use this webpage to install Outlook.   Unfortunately if you are using a Linux variant you will only be able to use the webpages.