Who needs coexistence?

Coexistence is the term we use to describe the existience and interaction of an on-premise system alongside a Microsoft Online service. The two key scenarios we support are

Exchange email coexistence
This allows an on-premise Exchange server (E2k or above) to exist side by side with Exchange Online. It enables you to move users from the on-premise Exchange server to Exchange Online at your own pace over several weeks or even months.

While doing so - users have a mailbox on either system, but retain their original @company.com email addresses and maintain a unified GAL that spans both systems.

Directory synchronization is a prequisite for this.

Directory Synchronization
Setting this up sets an ongoing synchronization from your on-premise Active Directory to Microsoft Online Services. This allows you to master user information in your Active Directory - and only manage license assignment and passwords up in Microsoft Online.

Change someone's Display Name? Do it on-premise and watch it sync up to Microsoft Online. Delete a user in Active Directory? The delete will propogate up to Microsoft Online.

Who needs coexistence?
Typically companies with over a 100 users. The benefits of using coexistence increase directly with the size of the company.

For companies with less than a 100 users, manually managing their users in the Microsoft Online Administraton Center might be acceptable. These companies are also often able to migrate from an on-premise system to Microsoft Online over a weekend, without the need for a phased migration of mail users.

For larger companies, migrating all mail users might take from a few weeks to a few months. Coexistence allows users to be moved across in phases by allowing both systems to work together.

Large companies also find keeping the two user directories in sync (on-premise Active Directory and Microsoft Online) painful, because of the volume of change they are likely to see.