Load balancing of outbound connections to remote domains is achieved by specifying more than one Hub Transport server in the same Active Directory site as a source server for the Send connector. Load balancing does not occur when the source servers for a Send connector are located in different Active Directory sites.
Note:
If the Hub Transport server is installed on the same hardware as the Mailbox server role, load balancing may not occur. When the Hub Transport server role is on the same hardware as the Mailbox server role, the local server is preferred for all messages that are sent by users who have mailboxes on that server. Therefore, in this scenario, true load balancing does not occur.
You cannot install the Hub Transport server role on the same hardware as the Mailbox server role when the Mailbox server role is deployed in a cluster.
However, we support deployment of the Hub Transport server role on the same hardware as the Client Access server role.
New in Exchange 2007 Service Pack 1
Exchange 2007 Service Pack 1 (SP1) supports the use of Network Load Balancing (NLB) to achieve high availability for specific types of connections to the Hub Transport server role. NLB can be used to provide high availability in the following scenarios:
Load balancing of inbound SMTP connections for POP and IMAP client connections to the default Receive connector named "Client
Load balancing of inbound SMTP connections for applications that submit e-mail to the Exchange organization.
NLB should not be used to distribute connections for internal routing between Hub Transport servers.
NLB is a good high availability option for both the Hub Transport server role and the Client Access server role when the Mailbox server role is deployed in a cluster.
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