For Internal Relay domains the Exchange servers behave like this:
If I have a local recipient within the organization with the SMTP address that the email is addressed to then deliver it to that mailbox. Otherwise, send it outside the organization.
Internal Relay domains are commonly used in shared SMTP namespace scenarios, where two separate mail systems both use the same domain name for email. If you want to know more about this scenario read How to Share an Email Domain Between Two Mail Systems.
The steps for setting up an Internal Relay domain are usually:
- Add the domain name to the Accepted Domains for the organization
- Create a Send Connector to route the non-local recipients in that domain to another external mail system
However the fact is that it will work just fine if you only do step 1, and let your main Send Connector for the “*” namespace (ie, all external domains) handle the routing outwards from the organization (either via smart host or DNS).
That is, unless you are using Edge Transport servers.
If you are using Edge Transport servers, have configured an Internal Relay domain, and have not configured a specific Send Connector for that namespace, you may see non-delivery messages when internal senders try to send to external recipients of that namespace.
This happens because an infinite loop is created between the Hub Transport and Edge Transport servers.
- The Hub Transport is correctly routing emails for non-local recipients in the Internal Relay domain name out of the organization via the Edge Transport servers.
- However the Edge Transport servers recognize the Internal Relay domain as being local to the organization, and therefore route the email back into the Hub Transport server (as they would if they'd received an email sent from an external sender and addressed to a recipient of that domain name).
Under those conditions you may see non-delivery reports for emails sent to non-local recipients of the Internal Relay domain.
In the diagnostic information will be the reason, an infinite loop.
#554 5.4.6 Hop count exceeded – possible mail loop ##
You will also see the loop in action in the message headers provided with the NDR.
The solution for this problem is to configure a Send Connector for the organization that is specifically for that Internal Relay domain name, that is a lower cost than the default Send Connector.
On an Exchange 2010 server in your organization (not the Edge Transport server) open the Exchange Management Console and navigate to Organization Configuration/Hub Transport. Select the New Send Connector task in the Actions pane of the console.
Give the Send Connector a name and click Next to continue.
Add the SMTP address space for the Internal Relay domain. Choose a cost that is lower than the default Send Connector that EdgeSync creates, which is a cost of 100 by default. Click Next to continue.
You can choose to route via DNS or a smart host, whichever suits your specific scenario. DNS is probably going to be fine if the MX records for that domain already point to where you want the mail to be routed to. Otherwise a smart host may be required. Click Next to continue.
Set the source server depending on which server you want to send out the emails to that domain. For Internal Relay domains the source server for the Send Connector must be a Hub Transport server, not an Edge Transport server, in order to achieve the desired email routing for all scenarios. This means that the Hub Transport server you choose must be able to make SMTP connections through your firewall to wherever it needs to route the email for the Internal Relay domain.
Finally, click New to complete the wizard and create the new Send Connector.
With the Send Connector in place you should see the correct routing behaviour in each scenario. Outside senders who send to a non-local recipient in the Internal Relay domain will be correctly routed into the Exchange organization first, and then back out the Send Connector from the Hub Transport server. Meanwhile email sent to local recipients of the Internal Relay domain will be delivered locally.
Email sent from internal senders to non-local recipients of the Internal Relay domain will be correctly routed out the Send Connector as well, while email sent to local recipients of the Internal Relay domain will be delivered locally as expected.
This configuration achieves the desired message delivery without infinite loop conditions.
Bottom line is, if you are using Internal Relay domains and also Edge Transport servers you must configure a Send Connector for handling non-local recipients in that domain, or else you will create an infinite loop condition.