Split Domain Routing: Getting Email for Your Domain at Two Providers

February 20th, 2013

Split Domain Routing (SDR) is a term used to describe how users with email addresses in the same domain can receive emails at two different email service providers.

For example, let’s say the company domain is “my-doctors-on-call.com,” and you are moving your email from Email Service Provider X to LuxSci. You have doctors Joe and Emily, whose email addresses are joe@my-doctors-0n-call.com and emily@my-doctors-on-call.com. The migration is incomplete or needs to be migrated over time, so Joe still needs to get his email at Email Service Provider X, but Emily is all moved and needs to get her email at LuxSci. This is called “Split Domain Routing.”

Why Split Domain Routing is Not Simple

To understand how SDR can be implemented, we need to take a step back and understand how email routing and delivery work in the first place.

Each domain has “DNS Records,” published on the internet and used to tell the world things such as what internet address your website is located at and what email servers receive inbound email for your domain. See Understanding Domain Name Service (DNS) to learn more about this.

Part of these DNS records are Mail Exchange (MX) records. These specify the email servers where inbound emails should be delivered. It’s possible to have more than one MX record, but generally they are all at the same provider and multiple definitions are used only for redundancy.

As all email for all users in the domain is delivered to the servers defined in the MX records without respect for WHO the email is destined for, Split Domain Routing cannot be enabled via any special DNS settings. Instead, you have to pick one of the companies and have ALL of your emails delivered there. So, any kind of re-routing of email needs to happen via special rules at one of your email service providers.

A Recipe for Split Domain Routing

Let’s take the scenario where my-doctors-on-call.com is migrating from Email Service Provider X to LuxSci and needs split domain routing to receive emails for some users at LuxSci before the transition is complete. Once the migration is complete, the DNS MX records for my-doctors-on-call.com will be updated to point to LuxSci’s MX records, and SDR will be no longer needed. However, before that, they need a protracted migration period for business reasons and require SDR.

Here is how it is set up:

  1. The DNS MX records for my-doctors-on-call.com will remain pointing to the inbound email servers of Email Service Provider X.
  2. LuxSci will set up a “subdomain” called “sdr.my-doctors-on-call.com” on its email system and configure it as a “domain alias.” Any email that arrives for emily@sdr.my-doctors-on-call.com is delivered on LuxSci to emily@my-doctors-on-call.com.
  3. DNS MX records for sdr.my-doctors-on-call.com will be created and point to LuxSci’s inbound email servers.
  4. The customer will create email forwarding rules in Email Service Provider X so that any inbound email messages that arrive at Email Service Provider X for any users that want to get their email at LuxSci are forwarded to those user’s addresses @sdr.my-doctors-on-call.com. For example, the customer would create a rule so that email to emily@my-doctors-on-call.com is forwarded to emily@sdr.my-doctors-on-call.com.
  5. Similar forwarding rules are created as more users are migrated to LuxSci and ready to use email there.
  6. Once everyone is migrated to LuxSci, the DNS MX records for my-doctors-on-call.com can be updated so that email is delivered directly to LuxSci, the subdomain can be deleted, and the account with Email Service Provider X can be closed.

Here is how this works:

  1. All email for @my-doctors-on-call.com arrives at Email Service Provider X.
  2. Email for selected users is forwarded to addresses @sdr.my-doctors-on-call.com.
  3. The DNS MX records direct that email to LuxSci.
  4. LuxSci recognizes these and delivers them locally to the respective LuxSci users @my-doctors-on-call.com.

This results in effective Split Domain Routing, where you can control exactly what addresses receive emails at which providers.

What are the requirements for Split Domain Routing to Work?

  1. You must be able to make MX records for subdomains in your DNS. Most DNS providers enable you to do this, but some restrict users from changing those settings.
  2. Your current provider must enable you to forward email messages. Most providers allow this.
  3. Your target provider must support subdomains that can work as “aliases” so that email to addresses in the subdomain can be delivered locally to the actual users in the main domain. LuxSci supports this, as do some other providers.

Are there any downsides to Split Domain Routing?

Clearly, adding SDR to your system adds complexity and should generally only be used as a temporary measure or in particular situations. Here are some issues that could crop up that you should be aware of when deciding whether to use SDR:

  1. Emails to users at the target provider have a longer way to go. They must be delivered to the first provider and then forwarded and delivered to the second provider. They may pass through multiple spam filters and other things. Generally, this is fine, but it could cause delays or delivery issues for some users that are not present for other, non-forwarded users.
  2. Email sent from users at the target company to users still at the old company may be delivered to the wrong place, depending on how the target company routes email. Some may automatically deliver the email to themselves or obey your DNS MX records. At LuxSci, this is not a problem. LuxSci delivers to your MX records and not locally, but it may be an issue with other companies.

What are other uses for Split Domain Routing?

There are many more applications for SDR than just migration to a new provider. These include:

Split Environment

Suppose some users require “Exchange,” and some require a different kind of service (like HIPAA-compliant email). In that case, you could use SDR to separate the Exchange users from the HIPAA users at a different provider. In this case, you would want the email delivered to the HIPAA provider first and the non-HIPAA split addresses forwarded (over SMTP TLS) to the exchange provider.

Backup Environment

A slight twist to Split Domain Routing is when you forward ALL emails for all users to the second provider but also allow these emails to be delivered to a primary provider. This causes a “fork” in your email flow where all email goes to two places. This can be useful if the primary provider goes down. You can log in to the secondary provider and get your emails.

We have several customers who use this approach where they

  1. Have a dedicated server in our premium environment for regular day-to-day email usage, and
  1. Have a shared email account in our basic environment that receives copies of their emails in this way. If the premium environment were ever offline, email can be accessed through our geographically separate environment.

Of course, one can achieve a similar level of redundancy with

  • Email Archival– to keep immutable copies of all inbound/outbound emails.
  • Email Continuity– a premium email filtering solution that spools your email in the case your inbound email servers are unavailable. It also allows you to send and receive emails in this case.

Which solution is better? A complete backup environment, continuity, archival, or both depends on your business and disaster recovery needs.

Have a question? Contact Us.