Announcing Exchange ActiveSync v16

One advantage of having your mailbox in Office 365 is that we usually deploy our innovations there first. We’re making enhancements to Exchange ActiveSync (EAS) in Office 365 that will soon be available to Exchange ActiveSync applications. Update: Exchange ActiveSync v16 will also be included in on-premise installations of Exchange Server 2016.

Updated EAS, which we will call version 16, contains three new major capabilities:

  1. The major focus is enhanced calendar reliability brought about through a reworking of the calendar workflow between server and client. This is a major benefit but ideally will be largely unnoticed by end-users. Success here will be the absence of those hard-to-diagnose calendar related problems sometimes seen when the server and client are not produced by the same company.
  2. Calendar attachments. Previously calendar items synced with EAS could not include any attachments such as agendas, decks or spreadsheets. In version 16 that is now possible.
  3. Syncing the drafts folder was previously not supported. We’ve added that support in version 16. Now you can start an email on your EAS device and continue editing that draft back at your desktop by retrieving the mail from the drafts folder. Or, write your draft at your computer, then make the final tweaks and send it from your phone while you're on the go.

Customers with their mailbox in Office 365 can take advantage of these new features when their favorite EAS client is updated. Update: We are happy to share that iOS 9, announced June 9 at Apple’s Worldwide Developer Conference, will support the calendar features of version 16 when it is released this fall.

As an end user, how can you tell if your mailbox is enabled for EAS version 16? The simplest way is to run an "Exchange ActiveSync" test at and look for version 16.0 listed as one of the supported protocol versions (MS-ASProtocolVersions header in the OPTIONS command response) in the test results. Note that you need a valid Office 365 email address and password in order to run that test, and the results are specific to that mailbox only (not the entire tenant).

Update: For partners who are building Exchange ActiveSync applications, preview documentation for EAS v16 is now available at

Note to developers: Exchange ActiveSync is different from the Office 365 Outlook REST APIs. Exchange ActiveSync is highly optimized for mobile clients connecting directly to Office 365/Exchange Online, and requires a licensing fee to use. The Outlook REST APIs are standards-based REST+JSON+OAuth Mail, Calendar, and Contact APIs that have no licensing fee, and are great for both mobile device-to-Office 365/Exchange Online, and service-to-Office 365/Exchange Online applications. We recommend that developers explore and evaluate the Outlook REST APIs for their applications.