Be able to fetch mails by "conversations" in folders in one request
Right now you cannot achieve "conversations" view in a mail folder using REST API with one request.
A workaround can be found here http://stackoverflow.com/questions/41161515/best-way-to-achieve-conversation-view-for-mail-folder-using-outlook-rest-api
but it is quite slow and not very efficient. Conversations view is the primary view of many mail client but is not achivable (easily) with REST API
We are aware of this. It is currently on our backlog. More votes would help prioritize this. ^JT