Understanding Shared and Split SMTP Namespaces for an Exchange 2007 Hybrid Deployment

Applies to: Exchange Server 2010 SP2

You can configure either a shared or split SMTP namespace when you configure a hybrid deployment between your on-premises Exchange organization and the cloud-based organization.

Shared SMTP Namespaces

A shared SMTP namespace, which is also known as a shared domain or shared address space, enables you to use the same e-mail domain address with both an existing on-premises Exchange organization and the cloud-based organization. A user with a mailbox in the cloud-based organization can have an e-mail address with the same domain as a user in the on-premises Exchange organization. For example, an on-premises user has an e-mail address of chris@contoso.com and a cloud-based user has an e-mail address of david@contoso.com.

When you configure a hybrid deployment between an on-premises Exchange organization and the cloud-based organization, a shared namespace is the default configuration. You choose the SMTP namespace that will be shared between the on-premises Exchange organization and the cloud-based organization when you add the chosen namespace to the cloud-based tenant as an accepted domain. The users in your on-premises Exchange organization and in the cloud-based organization will share that e-mail address domain.

On-Premises Address Policy Configuration

Most Exchange organizations have e-mail address policies that define the e-mail addresses that are applied to mailboxes and other recipients by default when they're created. The primary e-mail address domain is the domain all replies to a given recipient will be sent to. This approach is the same in hybrid organizations.

In hybrid organizations, an additional e-mail address domain must be added to the e-mail address policies in the on-premises Exchange organization. This e-mail address domain is the SMTP namespace of the cloud-based organization. For on-premises mailboxes, this additional address isn't used. However, when a mailbox is created in, or moved to the cloud-based organization, the on-premises Exchange organization uses this SMTP namespace as the target delivery address for those mailboxes. The examples in this checklist use service.contoso.com as the SMTP address of the cloud-based organization.

Shared Namespace Message Routing

With shared SMTP namespaces, your on-premises organization receives all e-mail addressed to the shared namespace, regardless of whether the recipient is in the on-premises organization or in the cloud-based organization. If you have an Exchange 2007 Edge Transport server configured, that server receives a message from the Internet. The Edge Transport server then sends the message to an on-premises Hub Transport server, which may be an Exchange 2007 server or the hybrid server. The Hub Transport server determines where to route the message, depending on the location of the recipient. This can be useful if you want to configure journaling, transport rules, anti-spam or antivirus policies that apply to all recipients in either organization.

When an e-mail message is received, a Hub Transport server resolves the recipient e-mail address on the message to a recipient object. If the recipient object is an on-premises mailbox or distribution group, the message is delivered to the recipient. If the recipient object is a mail user that's associated with a mailbox in the cloud-based organization, Exchange reviews the target delivery address of the mail user and redirects the message to the cloud-based organization. The message is passed to the hybrid server and is then delivered to the cloud-based organization and delivered to the cloud-based mailbox. See the following figure for an example of the message flow.

Inbound mail flow with a shared namespace

Learn more at: Understanding Transport Options

Split SMTP Namespaces

A split SMTP namespace is where your on-premises organization uses an SMTP namespace or domain that's different from the namespace used by the cloud-based tenant. Mailboxes in each organization will have e-mail addresses with different domains. For example, an on-premises user has an e-mail address of chris@contoso.com while a cloud-based user has an e-mail address of david@service.contoso.com.

As mentioned earlier, hybrid deployments use shared namespaces by default. If you want to use a split namespace, you must configure each mailbox manually.

On-Premises Address Policy Configuration

Most Exchange organizations have e-mail address policies that define the e-mail addresses that are applied to mailboxes and other recipients by default when they're created. The primary e-mail address domain is the domain all replies to a given recipient will be sent to.

As with organizations that use a shared namespace, you must add an additional e-mail address domain to the e-mail address policy in the on-premises Exchange organization. This e-mail address domain is the SMTP namespace of the cloud-based organization. For on-premises mailboxes, this additional address isn't used. However, when a mailbox is created in, or moved to the cloud-based organization, the on-premises Exchange organization uses this SMTP namespace as the target delivery address for those mailboxes.

Unlike organizations that use a shared namespace, you must manually remove the on-premises SMTP namespace and make the SMTP namespace of the cloud-based organization the primary SMTP namespace on the mailbox.

Split Namespace Message Routing

With split SMTP namespaces, messages that are sent to the on-premises SMTP namespace are sent to the on-premises Exchange organization, and messages sent to the cloud-based SMTP namespace are sent to the cloud-based organization. Messages to the cloud-based organization are never routed through the on-premises organization, even if recipients from both organizations are addressed on the same message. See the following figure for an example of the message flow.

Inbound mail flow with a split namespace

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