Set up split domain routing

  • Last updated on: 2016-01-18
  • Authored by: Rackspace Support

Split domain routing (SDR) allows you to have a single domain’s mailboxes distributed between our system and an external system, working together as if they were in one environment. This is primarily used when a large number of mailboxes are being migrated over to our system from another server over an extended period of time.

Mailboxes can be moved in batches to make the move more manageable for administrators and to make the transition almost seamless for the users. Though it is not common, SDR can also be used when you need to maintain some mailboxes on an existing system for an extended period of time or permanently.

If SDR will be a permanent feature of your set up they can point to either your external mail server or us. You may want to point your MX records to Rackspace because then all your messages get the benefit of our spam/anti-virus filtering instead of just the Rackspace-hosted mailboxes.

Setting up SDR

There are two parts to setting up SDR and they relate to enabling our system and the external system to have two-way communication. If you are using SDR as part of a migration, you will want to keep your MX records for the domain pointing to your existing server (described as your external mail server in this documentation). Once the migration is complete you can switch them to the Rackspace MX records listed here: Set up DNS records for Cloud Office email and Skype for Business.

Enabling SDR through the Cloud Office Control Panel

When SDR is enabled, our environment will forward to your external server any messages addressed to recipients on your domain that do not exist in our system. To enable it requires the hostname of your external mail server (usually one of the MX records that point to your external system) and a valid email address within the designated domain hosted on that external mail server. We will use this address only to validate the server. You must also open port 25 on the external mail server so that our system can connect to it.

To enable SDR

  1. Log in to the Cloud Office Control Panel.
  2. From the Go to section menu, select Domains.

  3. In the Tools section, click the Split Domain link.
  4. If prompted, select the domain for which you want to set up SDR.
  5. Select the Enable Split Domain Routing check box.
  6. In the External Mail Server box, enter the name of your external mail server.
  7. In the Verification Address box, enter in a valid email address that is hosted on your external mail server.
  8. Click Save.

Configuring SDR from your own external server to Rackspace

This type of functionality is known by several names including non-authoritative mail delivery or message routing for a shared address space. The idea is to set up the external server so that if it is not able to deliver a message locally, the server forwards to another system (Rackspace). Different mail systems will have their own procedures for configuring this.

The following Microsoft articles show how to accomplish this:

Subdomain message routing

This is a suggested way to route messages from your external server back to Rackspace. Subdomain routing uses contacts (or forwards) set up on your external system to route the messages to a subdomain email address. This same subdomain should be added as a domain alias on our system in order for this to work.

The following should be in place:

  • A subdomain created with your DNS host pointing to Rackspace MX records - for example, with its own set of MX records pointing to
  • Contacts (aliases) must be created on the external system for each mailbox hosted by Rackspace. The contact must forward the address to the address. For example, (on your external server) will have a forward set to Your server will see the address and query DNS, which will then resolve back to and deliver to the user in Rackspace’s environment.

  • Ensure that you have requested your subdomain as a domain alias. In order to do this, contact Support (chat, phone call, or open a ticket).

If Rackspace is your DNS host, enter this subdomain name in the Mail Records (MX) section of the DNS Settings page in the control panel.

Note: For a migration, when changing the MX records, ensure that you are changing them for the new subdomain (for example, and not the primary domain. After all mailboxes are on our system, you will change the MX records for the primary domain.

Share this information: