Subdomains: What are they, and do you need them?

Subdomains are “sub entries” under your domain name. Sometimes they’re more accurately called hostnames or FQDNs (“fully qualified domain names”) but for simplicity's sake, I’m mostly just going to call them subdomains here.

You're reading this on Spam Resource, which has a website address of, of which is the domain name. If I were going to create a subdomain for my email newsletter, I might choose In this case, is a subdomain of

If you send different types of email messages in any significant volume (at least thousands monthly), you might want to consider having separate subdomains for different from addresses for different types of emails that you send.

Let’s say you’re Jeremy Bonto, founder of the famous Bontocorp conglomerate, a company that sells a lot of widgets to people, and also has a lot of employees. It is potentially a good practice for Bontocorp to not send all emails with a from domain of, but instead to utilize different subdomains for from addresses for different types of mail. Like this:

  • Corporate mail: Send mail as <person>
  • Marketing messages: Send as <address>
  • Transactional messages: Send as <address>

What this does is allow you to build a separate domain reputation for each type of mail that you send. Meaning that if you have a deliverability problem with one type of mail, you reduce the chances that any issues will “bleed over” into another type of mail. Sending marketing messages too aggressively can make email from your marketing subdomain go to the spam folder at Gmail, but having a separate subdomain for transactional messages means that the chances of transactional mail being affected, when and if that happens, are reduced. (I think it mostly goes without saying that all of your mail should be authenticated properly with SPF and DKIM.)

This can help even if you’re sending all or most of this mail from a single IP address. (Whether or not you might want to use separate IP addresses is a separate question, and you have to meet volume requirements when sending via separate dedicated IP addresses. But for domains and subdomains, it's not quite as rigid.)

Similarly you might want to use separate subdomains for different email service providers, CRMs, or online store platforms you’re using. Not only might it simplify setup in various platforms, and of course it helps with segmentation of domain reputation, but it might even also make it easier to monitor your DMARC tool results to ensure everything is configured correctly in every platform you’re using to send mail.

If you're using Google for Business for your corporate email, you can even configure Google to treat the different subdomains as “user alias domains”, so that email to goes to the same address as This can be useful to make sure that inbound mail to addresses at your subdomains route to real people or to the right ticketing system.

Of course, none of this is a substitute for following best practices -- you still need to send wanted mail and not send spam -- but this is still a useful form of "inbox insurance" to help reduce the spread of a reputation-related deliverability issue if and when it were to occur. Let's face it, sometimes stuff happens, even to people who intend to do everything right.

And finally, I recommend against using totally different domains. Subdomains are great for security and consumer trust purposes -- if you send mail as and authenticated correctly, it's very clear to the recipient (and to mailbox providers) that the mail truly came from But any good guy or bad guy could register or When somebody gets an email from one of these other domains, often called lookalike or cousin domains, how do they really know that it came from the same entity as And if you DO manage to convince your subscribers that is really you, you're leaving open a gaping hole that could allow somebody to send as and pretend to be you, and maybe they'll be able to convince people they're legit, too, when they're not. This is bad news from a security best practices perspective.

I, myself, am guilty of guiding clients toward setting up "cousin" domains like this in the past. Sometimes configuration with an ESP platform can be complex enough that using a second domain makes the technical bits a lot easier to deal with. This was a much more common practice in the past, and you'll still run into some very legitimate folks sending mail this way. Inertia can be a powerful force, but please don't add to the problem by setting up domains this way anew, if you can avoid it.

Post a Comment