Internal Mail Routing

Mail is 'moved' by transport servers. Hub transport servers move email throughout the intranet and to other sites that belong to the organization. Edge transport servers send mail to the Internet.

Exchange uses the organizations Active Directory infrastructure to route mail between sites. For every site that a mailbox server role resides, you must have a hub transport server. The mailbox server role sends and receives all mail to and from a hub transport server in the same site. All mail that enters and leaves your organization must pass through the hub transport server.

Transport servers use connectors to shift mail. During installation default recieve and send connectors are created which allow messages to flow throughout the organization.They are good to go! No extra configuration is needed. Messages will be sent from one hub transport (in your site) to other hub transport servers in other sites where a mailbox server holds the recipients mailbox.

As explained, internal mail flow works with no extra configuration. The AD site infrastructure is held in the forests configuration partition (which is replicated to all DCs forest wide). It is this site infrastructure which is used to route mail. Active directory sites represent areas of well connected, high speed LANs. Sites are connected by intersite link objects. This class of object allows AD replication to occur every 3 hours by default. These can be assigned cost values which dictate preference of use. In the diagram NewYork can connect to Tokyo by first connecting to London or by connecting to Paris.
Active directory sites represent areas of well connected, high speed LANs. Sites are connected by intersite link objects. This class of object allows AD replication to occur every 3 hours by default. These can be assigned cost values which dictate preference of use.

In the diagram NewYork can connect to Tokyo by first connecting to London or by connecting to Paris. The total cost of the route is the sum of all the site link costs along the way. The route via London has a total cost of 20. Via Paris the total cost is 25. Messages from NewYork would be sent via London to recipients in Tokyo. AD administrators therefore define the message path since Exchange routing 'rides on the back ' of AD replication topology. This may be fine for your organization. However, Exchange administrators can take control of the situation and assign special 'Exchange costs' that override the normal AD site link costs. Note that Exchange administrators do not need any AD permissions for this. If Exchange costs of 5 are assigned to the site links that connect NewYork to Paris and Paris to Tokyo then messages from NewYork to Tokyo would now pass through London instead of Paris. AD replication would still travel via Paris. Exchange site link costs can be set by using the following cmdlet:

[PS] Set-ADSiteLink NewYork -ExchangeCost 5

To remove the site link cost set the -ExchangeCost value to $null
Consider the following scenario:

Messages are sent along the path that has the lowest accumulative cost (in this case NewYork - Paris - Dubai - Tokyo for messages sent from NewYork to Tokyo). You should remember that NewYork would try to contact Tokyo directly. What I mean is Exchange to Exchange communication. The NewYork exchange server would first attempt to send messages to the Tokyo exchange server first and would use the NewYork - Paris - Dubai - Tokyo routing path to get there. If it cant make this direct communication NewYork would attempt to contact the closest exchange server according to the routing topology. This would be Dubai. As you can see, messages do not relay to hub transports on route to the final destination server but backoff to a server in a site which is along the routing path. Messages are queued here in a retry state. This is called queue point of failure.
Another consideration is bifurcation. A message sent from someone in NewYork to multiple recipients in different sites would be sent by the NewYork server to a server where a fork in the routing path exists. The bifurcated message is therefore relayed to an Exchange server in a site that represents a fork in the individual recipients routing paths. This is called delayed fan-out.
Direct server to server relay between NewYork and Tokyo will not occur if a hub site exits along the routing path. A hub site is a AD site that messages would be sent to along the least cost routing path. What I mean here is all messages. Perhaps you require messages to be relayed to hub transport in a particular site first rather than the usual direct relay. However, the hub site designated must exist along the least cost routing path between source and destination.
You can designate an AD site as a hub site by using the following cmdlet

[PS] Set-ADSite Paris -HubSiteEnabled $true