Can an MX record and a CNAME record exist on the same subdomain?

You cannot have a CNAME and an MX record (or any other record type besides DNSSEC) on the exact same hostname. If sub.yourdomain.com is a CNAME, you need to either rely on the CNAME's target having MX records or use a different, unique subdomain for the MX record, such as mail.sub.yourdomain.com .

Does a subdomain used for sending emails need its own MX record?

Yes, it is highly recommended. For good email deliverability and reputation, any domain or subdomain used for sending emails should also be able to receive them. This signals legitimacy to recipient mail servers and allows for handling bounces and replies.

Will my email service provider's CNAMEs automatically include MX records?

In many cases, an email service provider's CNAMEs for verification might point to a hostname that already includes the necessary MX records. This means the ESP automates the MX setup for you. You should check your ESP's documentation or contact their support to confirm if this is the case for your specific setup.

What happens if an MX record is not properly configured for a sending subdomain?

Incorrect MX record configuration can lead to emails being rejected, sent to the spam folder, or even your domain being added to an email blocklist (also called a blacklist). Recipient mail servers use MX records to validate sending domains, and errors can negatively impact your sender reputation and cause mail delivery failures.

How can I troubleshoot MX record and CNAME conflicts on my subdomain?

Start by checking your DNS records for conflicts. If your subdomain has a CNAME, and you need an MX, consider using a unique subdomain for the MX record. Consult your ESP's documentation, as they might provide specific guidance or automate this. You can also use online DNS lookup tools to verify your records.

How to add an MX record to a subdomain when CNAMEs are used for email sending verification? - Technical - Email deliverability - Knowledge base

Setting up email sending infrastructure often involves a delicate dance with DNS records. One common hurdle arises when you need to add an MX record to a subdomain, especially if that subdomain is already using CNAMEs for email sending verification, such as for DKIM or tracking. This scenario can create a conflict because of how DNS works, leading to confusion and potential deliverability issues. It's a situation many email professionals encounter, and understanding the nuances is key to maintaining a healthy sending reputation and ensuring your emails reach the inbox.

The challenge stems from the fundamental rules governing CNAME records. While CNAMEs are excellent for aliasing one domain to another, they come with a significant restriction: a hostname that has a CNAME record cannot also have other types of records, like MX records. This can pose a problem when email service providers (ESPs) or certification bodies require specific DNS entries for verification or inbound mail routing on a subdomain that is already aliased.

Understanding DNS records for email

Understanding how various DNS records function is the first step to resolving configuration conflicts. MX records, or Mail Exchange records, are crucial for directing incoming mail to the correct mail server. When someone sends an email to example.com, the recipient's mail server looks up the MX record for example.com to find out where to deliver it. Without a proper MX record, inbound mail cannot be routed, which can affect replies to your sent emails or general domain reputation. A sending domain absolutely should have an MX record, even if it's just pointing to a server designed to handle bounces.

On the other hand, CNAME records are Canonical Name records, which create an alias from one domain name to another. They are widely used in email sending setups for authentication mechanisms like DKIM delegation, where your subdomain (e.g., sends.yourdomain.com) points to a hostname provided by your ESP (e.g., esp.service.com). This allows the ESP to manage the underlying DNS records for authentication, simplifying your setup. You can learn more about how CNAME delegation enables SPF and DKIM in your email sending practices.

The cname and other record conflict

The core problem arises because DNS standards dictate that if a domain or subdomain has a CNAME record, it cannot have any other resource records associated with it, except for DNSSEC related records. This means if marketing.yourdomain.com is a CNAME pointing to your ESP, you cannot directly add an MX record to marketing.yourdomain.com. This rule is in place to prevent ambiguities and ensure reliable DNS resolution. If you attempt to add an MX record to a hostname that is already a CNAME, DNS resolvers may behave unpredictably, leading to mail delivery failures or other unexpected issues. You can read more about subdomains that require both an MX record and a CNAME for further insights into this technical constraint.

Some email service providers automate the setup of MX records (and other critical records like SPF) when you use their CNAMEs for domain verification. This means their CNAME target (the domain your CNAME points to) is already configured with the necessary MX records, which then apply to your subdomain through the alias. However, not all ESPs do this, or the verification service (like ReturnPath or others) might have specific requirements that necessitate a direct MX record on the subdomain you are authenticating. This is where careful planning is required. In some cases, an MX record pointing to a CNAME alias's destination might technically work, but it's not considered best practice and can lead to less reliable email delivery.

Strategies for adding MX records

When you encounter the CNAME-MX conflict, you have a few strategies to consider. The most straightforward solution, if your ESP doesn't automatically handle MX records through their CNAMEs, is to use a slightly different subdomain for your MX record than the one used for CNAME verification. For instance, if sending.yourdomain.com is aliased via CNAME, you could set up an MX record on mail.sending.yourdomain.com. This separates the CNAME aliasing from the MX record, allowing both to coexist without conflict. However, this approach might not satisfy specific verification requirements that demand the MX record be on the exact same subdomain.

Another strategy is to check if your ESP (or the service requiring the MX record) allows for alternative verification methods or explicitly states how to handle MX records with CNAME-based setups. Some services might only need to see *an* MX record associated with the broader domain (e.g., yourdomain.com) rather than the specific subdomain. Always consult their documentation or support to ensure compliance. For a general understanding of DNS records for email sending, including those for subdomains, consider reading our guide on what DNS records are needed for sending subdomains.

Configuring the MX record

If you determine that you absolutely need an MX record on the same subdomain used for CNAME verification, and your ESP doesn't automate it, you might need to adjust your approach to CNAME usage for verification. In some rare cases, it might mean opting for direct TXT record verification instead of CNAME-based if available, though this removes the benefit of ESP-managed authentication records. Here is an example of a simple MX record you might add:

Example MX record for a subdomainDNS

Host: mail.subdomain.yourdomain.com
Type: MX
Priority: 10
Value: mailserver.your-esp.com

Remember, the hostname for the MX record should be the specific subdomain that needs to receive mail, or a unique subdomain created solely for mail routing, as per DNS standards regarding CNAME and MX record coexistence. If your primary domain (e.g., yourdomain.com) already has MX records for your main email service, adding an MX record to a subdomain will not interfere with them, as DNS records do not inherit from parent domains to subdomains.

Ensuring deliverability and preventing blocklisting

Proper DNS configuration for your email sending subdomains is fundamental to email deliverability. Incorrectly configured MX records, or conflicts between MX and CNAMEs, can lead to your emails being rejected, sent to spam folders, or even your domain being added to an email blocklist (or blacklist). Mail servers perform DNS lookups, including checking for MX records, to validate sending domains. If these lookups fail or return ambiguous results, it raises red flags that can negatively impact your sender reputation. A healthy sender reputation is crucial for consistent inbox placement.

Maintaining a clean sending reputation also involves ensuring your domain can receive mail. Even if you only use a subdomain for outbound marketing emails, having an MX record in place allows for the handling of bounces and replies, which signals to recipient mail servers that your domain is legitimate and actively managed. Neglecting this can make your domain look suspicious to spam filters, increasing the likelihood of your emails being flagged or even causing your domain to be put on a blocklist.

Monitoring and prevention

Regularly monitoring your email blocklist status and DMARC reports can provide early warnings about DNS issues affecting your email deliverability. DMARC reports, in particular, offer insights into SPF and DKIM authentication failures, which are often tied to DNS misconfigurations. If you're experiencing email deliverability challenges, these checks are an essential part of troubleshooting. Furthermore, setting up DMARC monitoring can help you detect and address problems proactively.

Views from the trenches

Use unique hostnames: If you have a CNAME for email sending verification (e.g., send.yourdomain.com), consider using a different subdomain for your MX record (e.g., mail.send.yourdomain.com) to avoid conflicts.
Check ESP documentation: Confirm if your email service provider automates MX record setup through their CNAMEs or has specific instructions for hybrid setups.
Maintain mail reception: Ensure any domain or subdomain used for sending mail can also receive mail, as this is a key signal for legitimate email activity.

Summary and best practices

Navigating DNS configurations, especially when dealing with the intricacies of MX records and CNAMEs on subdomains, can be complex. The key takeaway is that a CNAME record and any other record type, such as an MX record, cannot coexist on the exact same hostname. This fundamental DNS rule is crucial to prevent mail delivery issues and maintain the reliability of your email infrastructure.

By understanding the purpose of each record, consulting your ESP’s specific requirements, and if necessary, using distinct hostnames for CNAME-based verification and MX record setup, you can ensure your emails are both verified and deliverable. Always prioritize the ability of your sending domains to receive mail, as this directly impacts your sender reputation and keeps your emails out of the blocklist. Careful configuration and ongoing monitoring are essential for successful email deliverability.