Back to 'Deliverability'What is an email subdomain and when should you use one
Last updated:
When you send emails from your domain name, you probably use an email address like noreply@example.com. The problem starts when you send both newsletters and transactional emails from the same domain. A newsletter marked as spam damages the reputation of your entire domain. This can cause your password resets and order confirmations to no longer arrive or be seen as spam.
An email subdomain helps you separate different types of emails and protect your main domain from delivery problems. In this article, we explain what an email subdomain is, why developers use them, and when you should set up an email subdomain for your application or website.
What is an email subdomain
Before we dive into subdomains, let's first explain what a domain is. A domain is the unique address of your website, like lettermint.co. You use this domain for your website, but also for sending emails.
A subdomain is a prefix that comes before your main domain. For websites, you might recognize this from addresses like blog.example.com or docs. lettermint.co. These subdomains point to different parts of your website while remaining connected to your brand.
The same principle applies to email. An email subdomain looks like this:
noreply@mail.lettermint.co
In this example, mail is the subdomain. The email still comes from your domain, but email providers like Gmail and Outlook treat it separately from your main domain.
Root domain vs subdomain for email
The most important difference between your root domain (main domain) and a subdomain is that they have separate email reputations. Understanding root domain vs subdomain helps you determine the best strategy for your situation. Emails sent from noreply@example.com (root domain) and noreply@mail.example.com (subdomain) are treated differently by providers like Gmail and Outlook.
This means that problems with emails from your subdomain don't affect emails from your root domain, and vice versa. The distinction between root domain vs subdomain is crucial for developers to keep different types of emails separated and protect your main domain.
Email subdomains and sender reputation
Email providers track the reputation of every sending domain. This reputation determines whether your emails land in the inbox or spam folder. The most important thing to understand is that subdomains have their own reputation score, separate from your main domain.
This means that emails sent from newsletter@mail.example.com and noreply@example.com build different reputations. If your newsletter is marked as spam, it has no impact on emails sent from your main domain.
Why use an email subdomain
As we just wrote, subdomains have their own reputation, separate from your main domain. But why is that important? Email subdomains give you better control over your sender reputation and email delivery. These are the main reasons why developers use them:
1. Separate transactional emails from broadcast emails
Transactional emails like password resets and order confirmations are expected within seconds by users. These emails must always arrive, and preferably faster than light. Broadcast emails like newsletters typically have lower engagement. If these emails are marked as spam, it can damage your sender reputation.
By sending transactional emails from mail.example.com and newsletters from newsletter.example.com, you keep these reputations separate. Your important transactional emails remain reliable, even if a marketing campaign performs poorly.
2. Protect your main domain
You probably also use your main domain (example.com) for personal emails from your team and colleagues, like john@example.com or info@example.com. If a marketing campaign damages your main domain's reputation, these personal emails can also end up in spam. Especially during busy periods like [Black Friday and holidays](/knowledge-base/deliverability/prevent-emails-spam-holidays-gmail-outlook{title="Avoid spam during holidays"}, spam filters become stricter.
By using a subdomain for bulk sending, you protect your main domain. If something goes wrong with a campaign, only that specific subdomain is affected.
3. Easier troubleshooting
When you send all emails from one domain, it becomes difficult to determine what causes delivery problems. Are your marketing emails triggering provider spam filters, or is there a problem with your transactional emails?
4. Better organization for larger teams
If you offer multiple products or services, subdomains help keep everything organized. You might use app.example.com for application notifications, marketing.example.com for campaigns, and alerts.example.com for system messages. Each subdomain can be managed independently with its own sending patterns and volumes.
When should you use an email subdomain
Not every situation requires a subdomain. Here's when it makes sense to set one up:
- You send both transactional and broadcast emails.
If you send password resets and order confirmations alongside newsletters or promotions. - You send large email volumes.
With thousands or millions of emails, the risk of reputation damage increases. A subdomain gives you a safety net. If one subdomain has problems, your other email types continue working normally. - You're launching a new product or service.
Starting a new project under your existing domain? Use a subdomain from the start. - You have a staging or beta environment - Testing new features in a staging environment? Use a subdomain like staging.example.com or beta.example.com. This way test emails never affect your production reputation.
Tips for email subdomains
If you decide to use subdomains, follow these guidelines:
Choose clear, professional names
Use descriptive subdomain names that make sense. Good examples are:
- mail.example.com
- transactional.example.com
- newsletter.example.com
- notifications.example.com
Avoid confusing names like xyz.example.com or using completely different domains that don't match your brand. Recipients must immediately recognize that the email comes from your company.
Set up authentication correctly
Each subdomain needs its own SPF, DKIM and DMARC records. These protocols tell email providers that your subdomain is legitimate and help build trust.
At Lettermint, we automatically handle SPF and DKIM setup when you add a subdomain. You just need to add the DNS records we provide, we handle the rest.
Warm up new subdomains gradually
When you create a new subdomain, start by sending low volumes and increase gradually. This builds reputation slowly and prevents your emails from being marked as suspicious. Also read our article about greylisting to understand how email providers treat new senders.
Always test your new subdomain first with our test email addresses before sending to real users. This way you can see if everything works correctly without affecting your monthly usage.
Monitor the email statuses of your new subdomain carefully. Pay special attention to bounces and delayed statuses in the first weeks.
Use consistent sender addresses
Don't constantly switch between different email addresses on the same subdomain. Choose addresses like hello@mail.example.com or noreply@transactional.example.com and stick with them. Consistency helps build recognition with both email providers and recipients.
Setting up an email subdomain with Lettermint
Adding a subdomain to Lettermint is simple. Here's how it works:
- Choose your subdomain.
Choose something descriptive like mail.example.com - Add the subdomain to your Lettermint dashboard.
Go to Domains and add your new subdomain - Add DNS records to your domain.
We provide the exact DNS records you need to add to your subdomain. - Start sending.
Your subdomain is ready to use for sending emails!
For more information about setting up DNS records, check our documentation about adding a domain.
Conclusion
Email subdomains give you better control over your sender reputation and help keep your most important emails reliable. By separating transactional emails from broadcast emails, you prevent newsletter problems from affecting your important transactional emails.
Setting up a subdomain is simple and prevents problems in the future. Especially if you send both marketing emails and transactional emails, it's worth keeping them separate.