Set up split domain routing with an external mail server

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. You use this primarily when you are migrating a large number of mailboxes over to our system from another server over an extended period of time.

You can move mailboxes 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, you can also use SDR when you need to maintain some mailboxes on an existing system for an extended period of time or permanently.

If SDR is a permanent feature of your setup, they can point to either your external mail server or us. You might want to point your MX records to Thexyz because then all your messages get the benefit of our spam and anti-virus filtering instead of just the Thexyz-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 should keep your MX records for the domain pointing to your existing server (described as your external mail server in this documentation). After the migration completes, you can switch them to the Thexyz MX records listed here: Editing your domain DNS records at Thexyz

Split domain routing

Enabling SDR through Thexyz Email Admin Control Panel

When you enable SDR, our environment forwards to your external server any messages addressed to recipients on your domain that do not exist in our system. To enable it, you need 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 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.

Split domain routing

Enable SDR

  1. Log in to Thexyz Email Admin Control Panel.

  2. Select Domains.

  3. Click the Split Domain link.

Configuring SDR from your own external server to Thexyz

This type of functionality has 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 cannot deliver a message locally, the server forwards to another system (Thexyz). Different mail systems have their own procedures for configuring this.

The following Microsoft articles show how to accomplish this:

Subdomain message routing

We suggest that you use subdomain message routing to route messages from your external server back to Thexyz. Subdomain routing uses contacts (or forwards) set up on your external system to route the messages to a subdomain email address. YOu should add this same subdomain as a domain alias on our system in order for this to work.

Make sure you have the following elements in place:

  • A subdomain created with your DNS host pointing to Thexyz MX records: For example, with its own set of MX records pointing to
  • Contacts (aliases) created on the external system for each mailbox hosted by Thexyz. Your contact must forward the address to the address. For example, (on your external server) should have a forward set to Your server then sees the address and query DNS, which resolves back to and delivers to the user in the Thexyz environment.
  • Ensure that you have requested your subdomain as a domain alias. To do this, contact Support (chat, phone call, or open a ticket).

Split domain routing

If Thexyz 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 (such as, and not the primary domain. After all mailboxes are on our system, you should change the MX records for the primary domain.

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