When you choose to use a shared namespace, all recipients in the on-premises and cloud-based organizations share the same SMTP domain in their e-mail addresses. The mail exchanger (MX) record for this SMTP domain sends mail to the on-premises Exchange organization.

When a message arrives at the on-premises Exchange organization for a recipient that resides in the cloud, the on-premises Exchange 2010 hybrid server will automatically redirect that message to the cloud-based recipient. The hybrid server determines whether a mailbox is located on an on-premises Exchange server or in the cloud-based organization by checking the recipient type. If the recipient type is a mailbox, the hybrid server routes the message to the on-premises Exchange server that contains that mailbox. If the recipient type is a remote mailbox, which is a special type of mail user, the hybrid server retrieves the remote routing address for that remote mailbox. The remote routing address for the mail user is the SMTP address of its associated mailbox in the cloud-based organization. The hybrid server readdresses the message with the SMTP address of the cloud-based mailbox and sends the message to the cloud-based organization. The examples in this checklist use service.contoso.com as the SMTP address of the cloud-based organization.

Important:
You must not use the service tenant FQDN, for example, contoso.onmicrosoft.com, as the SMTP address of the cloud-based organization.

Note:
For the best experience when you deploy a hybrid organization, we strongly recommend that you use a shared namespace.

When you choose to use a split namespace, the e-mail addresses of recipients in the cloud-based organization are configured with an SMTP domain that's different from e-mail addresses of recipients in the on-premises organization. Messages sent to recipients in one organization are delivered directly to that organization.

Learn more about shared and split namespaces at: Understanding Shared and Split SMTP Namespaces

In addition to choosing how inbound messages addressed to recipients to your organizations are routed, you can also choose how outbound messages sent from cloud-based recipients are routed. The following describes the available options:

• Centralized mail control   This option routes outbound messages sent from the cloud-based organization through your on-premises organization. Except for messages sent to other recipients in the same cloud-based organization, all messages sent from recipients in the cloud-based organization are sent through the on-premises organization. This enables you to apply compliance rules to these messages and any other processes or requirements that must be applied to all of your recipients, regardless of whether they're located in the cloud-based organization or the on-premises organization.
  
  

  Important:
  Your on-premises hybrid server must be accessible from the Internet for recipients in the cloud-based organization to send messages to the Internet. If your on-premises hybrid server is unavailable, messages sent from the cloud-based organization will queue until it's available again.

• Decentralized mail control   This option routes outbound messages sent from the cloud-based organization directly to the Internet. Use this option if you don't need to apply any on-premises policies or other processing to messages that are sent from recipients in the cloud-based organization.
  
  

Regardless of whether you've selected shared or split namespaces, or centralized or decentralized mail control, all messages that are sent between recipients in your on-premises organization and the cloud-based organization are sent directly to and from either organization. As part of the configuration provided in the procedures in this checklist, each organization is configured to treat messages sent from the other organization as internal. This allows messages to bypass anti-spam settings and other services.

To help protect recipients in both organizations, and to help ensure that messages sent between the organizations aren't intercepted and read, transport between both organizations is configured to use forced TLS transport using Secure Sockets Layer (SSL) certificates provided by a trusted third-party Certificate Authority (CA).

When using forced TLS transport, the sending and receiving servers examine the certificate configured on the other server. The subject name, or one of the subject alternative names (SANs), configured on the certificates must match the fully qualified domain name (FQDN) that an administrator has explicitly specified on the other server. For example, if the cloud-based organization is configured to accept and secure messages sent from the mail2.contoso.com FQDN, the sending on-premises hybrid server must have an SSL certificate with mail2.contoso.com in either the subject name or SAN. If this requirement isn't met, the connection is refused.

Note:
The FQDN used doesn't need to match the e-mail domain name of the recipients. The only requirement is that the FQDN in the certificate subject name or SAN must match the FQDN that the receiving or sending servers are configured to accept.

Learn more about SSL certificates and domain security at: Understanding Certificate Requirements, Understanding TLS Certificates

When you configure your on-premises and cloud-based organization to use a shared namespace and to also use centralized mail control, all messages sent to and from recipients in both the on-premises organization and the cloud-based organization are sent through the on-premises organization.

In the diagram below, which shows inbound messages sent to recipients in your organization, the following occurs:

1. An inbound message is sent from an Internet sender to the recipients chris@contoso.com and david@contoso.com. Chris's mailbox is located on an Exchange 2003 server in the on-premises organization. David's mailbox is located in the cloud-based organization.
   
   
2. Because the recipients both have contoso.com e-mail addresses, and the MX record for contoso.com points to the on-premises hybrid server, the message is delivered to the on-premises hybrid server.
   
   
3. The hybrid server performs a lookup for each recipient using an on-premises global catalog server. Through the global catalog lookup, it determines that Chris's mailbox is located on the Exchange 2003 server while David's mailbox is located in the cloud and has a routing address of david@service.contoso.com
   
   
4. The hybrid server splits the message into two copies. One copy of the message is sent through the routing group connector that's configured between the hybrid server and the Exchange 2003 server and is delivered to Chris's mailbox.
   
   
5. The second copy is sent through the send connector that's configured between the hybrid server, over the Internet, and to the Forefront Online Protection for Exchange (FOPE) service, which receives message sent to the cloud-based organization.
   
   
6. FOPE scans the message for viruses and then sends the message to the cloud-based organization where the message is delivered to David's mailbox.
   
   

In the diagram below, which shows outbound messages sent to the Internet, the following occurs:

1. Chris, who has a mailbox on the on-premises Exchange 2003 server, sends a message to an external Internet recipient, erin@cpandl.com. David, who has a mailbox in the cloud-based organization, sends a message to the external recipient brian@cpandl.com. Both Chris and David have a contoso.com reply address.
   
   
2. The Exchange 2003 server sends Chris's message through a routing group connector to the hybrid server.
   
   
3. The cloud-based organization sends David's message to FOPE.
   
   
4. FOPE is configured to send all Internet-bound messages to the on-premises hybrid server, so the message is routed to the hybrid server.
   
   
5. The hybrid server performs compliance, anti-virus, and any other processes configured by the administrator, on both Chris's and David's messages.
   
   
6. The hybrid server looks up the MX record for cpandl.com and sends the message to the cpandl.com mail servers located on the Internet.
   
   

When you configure your on-premises and cloud-based organizations to use a shared namespace, but choose to use decentralized mail control, all inbound messages sent to recipients in either organization are sent through the on-premises organization. However, outbound messages sent from recipients in either organization are sent directly to the Internet. The cloud-based organization doesn't send messages to the Internet through the on-premises organization.

In the diagram below, which shows inbound messages sent to recipients in your organization, the following occurs:

1. An inbound message is sent from an Internet sender to the recipients chris@contoso.com and david@contoso.com. Chris's mailbox is located on an Exchange 2003 server in the on-premises organization. David's mailbox is located in the cloud-based organization.
   
   
2. Because the recipients both have contoso.com e-mail addresses, and the MX record for contoso.com points to the on-premises hybrid server, the message is delivered to the on-premises hybrid server.
   
   
3. The hybrid server performs a lookup for each recipient using an on-premises global catalog server. Through the global catalog lookup, it determines that Chris's mailbox is located on the Exchange 2003 server while David's mailbox is located in the cloud and has a routing address of david@service.contoso.com
   
   
4. The hybrid server splits the message into two copies. One copy of the message is sent through the routing group connector that's configured between the hybrid server and the Exchange 2003 server and is delivered to Chris's mailbox.
   
   
5. The second copy is sent through the Send connector that's configured between the hybrid server, over the Internet, and to FOPE, which receives messages sent to the cloud-based organization.
   
   
6. FOPE scans the message for viruses and then sends the message to the cloud-based organization where the message is delivered to David's mailbox.
   
   

In the diagram below, which shows outbound messages sent to the Internet, the following occurs:

1. Chris, who has a mailbox on the on-premises Exchange 2003 server, sends a message to an external Internet recipient, erin@cpandl.com. David, who has a mailbox in the cloud-based organization, sends a message to the external recipient brian@cpandl.com. Both Chris and David have a contoso.com reply address.
   
   
2. The Exchange 2003 server sends Chris's message through a routing group connector to the hybrid server.
   
   
3. On the hybrid server, compliance, anti-virus, and any other processes configured by the administrator, are applied to Chris's message.
   
   
4. The hybrid server then looks up the MX record for cpandl.com and sends the message to the cpandl.com mail servers located on the Internet.
   
   
5. The cloud-based organization sends David's message to FOPE.
   
   
6. FOPE is configured to send all Internet-bound messages directly to the Internet. FOPE looks up the MX record for cpandl.com.
   
   
7. FOPE delivers the message directly to the cpandl.com mail servers located on the Internet. Because the message never transits through the hybrid server, no on-premises processes are applied to it.
   
   

In the diagram below, which shows inbound messages sent to recipients in your organization, the following occurs:

1. An inbound message is sent from an Internet sender to the chris@contoso.com and another message is sent to david@service.contoso.com. Chris's mailbox is located on an Exchange 2003 server in the on-premises organization. David's mailbox is located in the cloud-based organization.
   
   
2. Because the recipients have different e-mail address domains, the sending server sends each message to the organization that receives messages for each domain. The MX record for contoso.com points to the on-premises hybrid server while the MX record for service.contoso.com points to FOPE.
   
   
3. The message for Chris is sent to the hybrid server. Through the global catalog lookup, it determines that Chris's mailbox is located on the Exchange 2003 server.
   
   
4. The hybrid server sends the message through the routing group connector that's configured between the hybrid server and the Exchange 2003 server and the message is delivered to Chris's mailbox.
   
   
5. The message for David is sent to FOPE, which receives message sent to the cloud-based organization.
   
   
6. FOPE scans the message for viruses and then sends the message to the cloud-based organization where the message is delivered to David's mailbox.
   
   

In the diagram below, which shows outbound messages sent to the Internet, the following occurs:

1. Chris, who has a mailbox on the on-premises Exchange 2003 server, sends a message to an external Internet recipient, erin@cpandl.com. David, who has a mailbox in the cloud-based organization, sends a message to the external recipient brian@cpandl.com. Chris has a reply address of chris@contoso.com and David has a reply address of david@service.contoso.com.
   
   
2. The Exchange 2003 server sends Chris's message through a routing group connector to the hybrid server.
   
   
3. The cloud-based organization sends David's message to FOPE.
   
   
4. FOPE is configured to send all Internet-bound messages to the on-premises hybrid server, so the message is routed to the hybrid server.
   
   
5. The hybrid server performs compliance, anti-virus, and any other processes configured by the administrator, on both Chris's and David's messages.
   
   
6. The hybrid server then looks up the MX record for cpandl.com and then sends the message to the cpandl.com mail servers located on the Internet.
   
   

In the diagram below, which shows inbound messages sent to recipients in your organization, the following occurs:

1. A message is sent from an Internet sender to chris@contoso.com and another message is sent to david@service.contoso.com. Chris's mailbox is located on an Exchange 2003 server in the on-premises organization. David's mailbox is located in the cloud-based organization.
   
   
2. Because the recipients have different e-mail address domains, the sending server sends each message to the organization that receives messages for each domain. The MX record for contoso.com points to the on-premises hybrid server while the MX record for service.contoso.com points to FOPE.
   
   
3. The message for Chris is sent to the hybrid server. Through the global catalog lookup, it determines that Chris's mailbox is located on the Exchange 2003 server.
   
   
4. The hybrid server sends the message through the routing group connector that's configured between the hybrid server and the Exchange 2003 server, and the message is delivered to Chris's mailbox.
   
   
5. The message for David is sent to FOPE, which receives message sent to the cloud-based organization.
   
   
6. FOPE scans the message for viruses and then sends the message to the cloud-based organization where the message is delivered to David's mailbox.
   
   

In the diagram below, which shows outbound messages sent to the Internet, the following occurs:

1. Chris, who has a mailbox on the on-premises Exchange 2003 server, sends a message to an external Internet recipient, erin@cpandl.com. David, who has a mailbox in the cloud-based organization, sends a message to the external recipient brian@cpandl.com. Chris has a reply address of chris@contoso.com while David has a reply address of david@service.contoso.com.
   
   
2. The Exchange 2003 server sends Chris's message through a routing group connector to the hybrid server.
   
   
3. The hybrid server performs compliance, anti-virus, and any other processes configured by the administrator, on Chris's message.
   
   
4. The hybrid server then looks up the MX record for cpandl.com and sends the message to the cpandl.com mail servers located on the Internet.
   
   
5. The cloud-based organization sends David's message to FOPE.
   
   
6. FOPE is configured to send all Internet-bound messages directly to the Internet. FOPE looks up the MX record for cpandl.com.
   
   
7. FOPE delivers the message directly to the cpandl.com mail servers located on the Internet. Because the message never transits through the hybrid server, no on-premises processes are applied to it.