Shared Address Space with On-Premises Relay

Ironically, to configure a single shared address space, you need to configure multiple domains. The following domains are required for a shared address space:

• The domain for the shared address space itself   In this example, the shared domain is @contoso.edu. This is also the domain that is used for the on-premises messaging system.
  
• A specific domain for mailboxes in the cloud-based email service   In this example, the cloud-based domain is @live.contoso.edu.
  

The cloud-based domain must be different from the on-premises domain so email is correctly routed between the on-premises messaging system and the cloud-based email service. Senders and recipients that are outside the organisation don't care about the cloud-based domain, but it is a vital part of making the shared address space work correctly.

A key ingredient to a shared address space is correctly configuring the email addresses on mailboxes in the on-premises messaging system and in Outlook Live.

Here's how the email addresses must be configured on all mailboxes:

• The primary address   The primary address is used as the From: address for all messages sent from the mailbox. There can be only one value for the primary address. In this example, everyone's primary address is in the shared address space @contoso.edu.
  
• Proxy addresses   Proxy addresses are additional addresses for a mailbox. Proxy addresses are also known as secondary email addresses. The mailbox can receive email sent to any of its proxy addresses. The primary address is always listed as a proxy address.
  

Here are the correct values for the primary address and proxy addresses for on-premises mailboxes and Outlook Live mailboxes.

As noted earlier, the on-premises messaging system is configured to accept all incoming email from the Internet for the shared address space. In the Contoso University example, all email for the @contoso.edu domain is delivered to the on-premises messaging system. You accomplish this by configuring the MX record for the contoso.edu domain in an Internet-facing DNS server to point to the on-premises messaging system.

After the email arrives, the on-premises messaging system is responsible for correctly determining if the recipient has a mailbox in the on-premises messaging system or in the cloud-based email service, and then delivering the message or forwarding the message as appropriate.

Here are two email routing scenarios in a shared address space:

• Email sent to students in the cloud-based email service   The messages could come from external senders on the Internet, or from faculty and staff in the on-premises messaging system. The on-premises messaging system is configured to forward email for students to the cloud-based email service. The required configuration depends heavily on the nature of the on-premises messaging system. For details, see Configure a Shared Address Space with On-Premises Relay.
  
• Email sent from students in the cloud-based email service to faculty and staff in the on-premises messaging system   The shared address space @contoso.edu is configured as an internal relay domain in the cloud-based email service. When the faculty or staff recipient isn't found in the cloud-based shared address book, the message is routed to the Internet and the contoso.edu domain points to the on-premises messaging system, so the message is delivered successfully.
  

Internal email between recipients in the on-premises messaging system or between students in the cloud-based email service is straight-forward. In both cases, the recipients are in their respective address books, so the message is delivered locally.

Likewise, outgoing email to recipients outside the organisation operates as expected. The on-premises messaging system uses its existing path to the Internet to deliver email messages to the Internet for recipients outside the organisation, and the cloud-based email service delivers messages directly to the Internet.