Nylas currently connects Microsoft accounts (like Office 365, Exchange or Outlook.com) using a mix of protocols like Exchange ActiveSync (EAS), Exchange Web Services (EWS) and more recently Microsoft Graph but we try to hide this detail away from customers and end-users.
Following on from that, some Nylas API endpoints like /free-busy or /availability only work when the account is connected with EWS. To ensure a Microsoft account gets connected with EWS, you can add the calendar.free_busy scope when authenticating accounts. This will force the account to connect to EWS and the authentication will fail if it cannot. Otherwise, Nylas falls back to connecting with Exchange ActiveSync where these endpoints won't work.