Exchange ActiveSync client connectivity in Office 365

This article explains how mobile devices connect to Exchange Online (Office 365) service and how the connectivity may be impacted if the device does not support certain Exchange ActiveSync (EAS) protocol requirements.

Exchange ActiveSync protocol versions

Most mobile devices that connect to Exchange do so using the Exchange ActiveSync protocol. Each successive version of the protocol offers new capabilities. (The Exchange ActiveSync article maintained by the Exchange community on Wikipedia has more details. -Editor)

Before any device accesses an Exchange mailbox, it negotiates with the Exchange server to determine the highest protocol version that they both support, and then uses this protocol version to communicate. Through the protocol version negotiation, the device and the server agree to behave in a particular manner in accordance with the version selected.

Mailbox redundancy in Office 365

In Office 365, we store multiple copies of user mailboxes, geographically distributed across different sites and datacenters. This redundancy ensures that if one copy of the mailbox fails for some reason (for example due to a hardware failure on a particular server), we can access the same mailbox elsewhere. At any given time, one copy of a particular mailbox is considered active and the remaining ones are deemed passive. When a user connects to their mailbox, they take actions on the active copy, and changes are then propagated to its passive copies.

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