You need to consider the impact of configuring an existing Exchange 2010 Hub Transport server in your Exchange organization as the hybrid server. Here are some things to think about:

• Exchange 2010 service pack   All the Exchange 2010 servers in the site where you're configuring the hybrid server must be running, at minimum, Exchange 2010 Service Pack 1 (SP1)
  
  
• Message routing   All messages sent to and from Exchange 2010 mailboxes are handled by the Exchange 2010 Hub Transport servers in the organization. However, messages sent to and from the cloud-based organization are handled by the hybrid server. Messages sent to Internet recipients can be routed directly between the hybrid server and the Exchange 2010 Edge Transport server, if one is configured.
  
  

Learn more at: Understanding Transport in a Hybrid Deployment

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 Edge Transport server determines whether the message is spam or is malicious and, if not, forwards it to a Hub Transport server in your organization. The message can be forwarded to any Hub Transport server in an Active Directory site, including the hybrid server.

The Hub Transport 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 Hub Transport 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 Hub Transport 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 Hub Transport server readdresses the message with the SMTP address of the cloud-based mailbox. If the Hub Transport server that performed the lookup is not the hybrid server, the server sends the message to the hybrid server. The hybrid server then 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 hybrid deployment experience, 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 mail.contoso.com FQDN, the sending on-premises hybrid server must have an SSL certificate with mail.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.

Trusted communication between your on-premises organization and cloud-based organization requires that the on-premises server accepting the connection, called the TLS endpoint, be an Exchange 2010 server. In your on-premises organization, this can be the hybrid server or any other Exchange 2010 Hub Transport or Edge Transport server. If the TLS endpoint is a non-Exchange 2010 server, the connection will fail. The instructions in this checklist configure the hybrid server as the TLS endpoint. This requires that you provide an external IP address to the hybrid server and open port 25 on your firewall to the hybrid server.

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 2010 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 Edge transport server, the message is delivered to the on-premises Edge Transport server.
   
   
3. The Edge Transport server selects a Hub Transport server in the on-premises organization to transfer the message to. Because the hybrid server has the Hub Transport server role installed, the message is sent to the hybrid server.
   
   
4. The message is delivered to the hybrid server which 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 2010 server while David's mailbox is located in the cloud and has a routing address of david@service.contoso.com.
   
   
5. The hybrid server splits the message into two copies. One copy of the message is sent to Chris's Exchange 2010 mailbox.
   
   
6. The second copy of the message is sent over the Internet through the Send connector that's configured between the hybrid server and the Forefront Online Protection for Exchange (FOPE) service, which receives message sent to the cloud-based organization.
   
   
7. 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 2010 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 2010 mailbox server sends Chris's message to the hybrid server because it has the Hub Transport server role installed. The hybrid server sends the message to the Exchange 2010 Edge Transport 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. FOPE is configured to bypass the on-premises Exchange 2010 Edge Transport server.
   
   
5. The hybrid server sends the message to the Exchange 2010 Edge Transport server.
   
   
6. The Edge Transport server performs compliance, anti-virus, and any other processes configured by the administrator, on both Chris and David's messages.
   
   
7. The Edge Transport server looks up the MX record for cpandl.com and sends the messages 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 2010 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 Edge transport server, the message is delivered to the on-premises Edge Transport server.
   
   
3. The Edge Transport server selects a Hub Transport server in the on-premises organization to transfer the message to. Because the hybrid server has the Hub Transport server role installed, the message is sent to the hybrid server.
   
   
4. The message is delivered to the hybrid server which performs a lookup for each recipient using an on-premises global catalog server. Through the global catalog lookup, the hybrid server determines that Chris's mailbox is located on the Exchange 2010 server while David's mailbox is located in the cloud and has a routing address of david@service.contoso.com.
   
   
5. The hybrid server splits the message into two copies. One copy of the message is sent to Chris's Exchange 2010 mailbox.
   
   
6. The second copy of the message is sent, over the Internet, through the Send connector that's configured between the hybrid server and the Forefront Online Protection for Exchange (FOPE) service, which receives message sent to the cloud-based organization.
   
   
7. 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 2010 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 2010 mailbox server sends Chris's message to the hybrid server because it has the Hub Transport server role installed. The hybrid server sends the message to the Exchange 2010 Edge Transport server.
   
   
3. The Edge Transport server performs compliance, anti-virus, and any other processes configured by the administrator, on Chris's message.
   
   
4. The Edge Transport server 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 2010 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 Edge Transport server while the MX record for service.contoso.com points to FOPE.
   
   
3. The Edge Transport server sends the message to the hybrid server because it has the Hub Transport server role installed.
   
   
4. 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 2010 server.
   
   
5. The hybrid server delivers the message to Chris's mailbox on the Exchange 2010 server.
   
   
6. The message for David is sent to FOPE, which receives message sent to the cloud-based organization.
   
   
7. 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 2010 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 2010 mailbox server sends Chris's message to the hybrid server which has the Hub Transport server role installed. The hybrid server sends the message to the Exchange 2010 Edge Transport 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. FOPE is configured to bypass the on-premises Exchange 2010 Edge Transport server.
   
   
5. The hybrid server sends the message to the Exchange 2010 Edge Transport server.
   
   
6. The Edge Transport server performs compliance, anti-virus, and any other processes configured by the administrator, on both Chris and David's messages.
   
   
7. The Edge Transport server looks up the MX record for cpandl.com and sends the messages 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. An inbound 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 2010 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 Edge Transport server while the MX record for service.contoso.com points to FOPE.
   
   
3. The Edge Transport server sends the message to the hybrid server because it has the Hub Transport server role installed.
   
   
4. The hybrid server performs a lookup for each recipient using an on-premises global catalog server. Through the global catalog lookup, the hybrid server determines that Chris's mailbox is located on the Exchange 2010 server.
   
   
5. The hybrid server delivers the message to Chris's mailbox on the Exchange 2010 server.
   
   
6. The message for David is sent to FOPE, which receives message sent to the cloud-based organization.
   
   
7. 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 2010 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 2010 mailbox server sends Chris's message to the hybrid server because it has the Hub Transport server role installed. The hybrid server sends the message to the Exchange 2010 Edge Transport server.
   
   
3. The Edge Transport server performs compliance, anti-virus, and any other processes configured by the administrator, on Chris's message.
   
   
4. The Edge Transport server 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.