Configure Subdomain Delegation for AWS SES

Why subdomain delegation matters for AWS SES sending domains

You’ve set up AWS SES to send transactional emails from your custom domain—but your messages are bouncing, or worse, landing in spam folders without explanation. The issue might not be your content. It’s likely a missing DNS step: subdomain delegation.

When you send email through AWS SES using a custom domain, you must assign a dedicated subdomain (like mail.yourdomain.com) to handle authentication and reputation tracking. Without proper DNS delegation, your email’s SPF alignment fails, and major providers see your sender as untrusted.

Think of it like a building with multiple tenants—each with their own security key. If you don’t assign a unique key (DNS record) to each tenant (subdomain), the building’s security system (email providers) assumes someone’s impersonating the owner. That’s how you end up blocked.

Key takeaways

Subdomain delegation isolates sending reputation between different email streams in AWS SES
Skipping subdomain delegation causes SPF alignment failures and impacts inbox placement
Delegating mail.yourdomain.com to AWS SES ensures correct DKIM and SPF alignment

What is subdomain delegation for AWS SES?

You can use a subdomain like mail.yourdomain.com as a dedicated sending domain for AWS SES by delegating it through DNS, which isolates SES traffic from your primary domain. This setup prevents issues on your main domain (like spam complaints or blacklisting) from impacting all your email sends. It’s a standard practice for scaling email volume while maintaining sender reputation.

How subdomain delegation works

When you configure subdomain delegation, you create DNS records that point your subdomain to AWS SES’s infrastructure—without touching your root domain’s DNS. This means no changes to your main domain’s SPF, DKIM, or DMARC policies. The result is a clean separation: your primary domain stays protected, while your subdomain handles transactional or bulk email.

For example, if you send order confirmations via mail.yourcompany.com, AWS SES only sees that subdomain as legitimate. If abuse happens, the damage is contained. That’s not just best practice—it’s how large senders like Amazon, Netflix, and financial institutions structure their email systems at scale.

Why it matters for deliverability and reputation

Using a subdomain keeps the reputation of your main domain untainted. High-volume email campaigns can trigger spam filters, especially if the list quality dips. If you’re sending from yourdomain.com and that spikes, your whole domain risks blacklisting. But sending from a subdomain? You’re limiting the exposure.

As the Messaging, Malware, and Mobile Anti-Abuse Working Group (M3AAWG) notes, consistent sender identity and clear separation of traffic improve inbox placement. The same principle applies to email verification: you’re more likely to succeed with clean, verified addresses sent from a dedicated subdomain.

With MailTester, you can validate your list before sending—ensuring only valid addresses reach your subdomain. A high-quality list reduces hard bounces and spam complaints, which are critical for maintaining strong deliverability. Try verifying your email list before sending to AWS SES to confirm accuracy and reduce risk.

Step-by-step: Configure subdomain delegation for AWS SES

You can configure subdomain delegation for AWS SES by verifying your subdomain (like mail.yourdomain.com) in the SES console, adding a required CNAME record to your DNS provider, waiting for propagation, and confirming the record in AWS. Once complete, AWS SES treats the subdomain as a dedicated sending identity, improving sender reputation and inbox placement. This setup is standard practice for high-volume senders.

Set up the subdomain identity in AWS SES

Log in to the AWS Management Console and go to the Amazon SES service. This is where you manage sending identities and verify domain ownership.
Navigate to Verified Identities and click Add another identity. You’ll see a list of identity types; choose Email address to verify a subdomain.
Enter your subdomain, such as mail.yourdomain.com. AWS will treat this as an identity separate from your root domain, allowing granular control over sending permissions and reputation.
Copy the CNAME record AWS generates. This record proves you control the subdomain and is required for verification. Do not modify the name or value.

Apply the CNAME record in your DNS provider

Go to your DNS provider’s control panel (Cloudflare, GoDaddy, Route 53, etc.). The exact steps vary by provider, but you’ll find a section for adding DNS records.
Add the CNAME record with the exact name and value provided by AWS. Accuracy is critical—any typo breaks verification.
Wait for DNS propagation. Most changes take under 1 hour, but DNS caching can delay this. Use tools like MXToolbox to check if the record is live.
Return to AWS SES and click Verify next to your subdomain. AWS checks the CNAME record in real time.
Once verified, enable sending for the subdomain. You can now send mail from addresses like [email protected] with full alignment and better deliverability.

Subdomain delegation helps isolate sending activity from other domains, which improves deliverability. It’s an industry-standard practice used by large organizations to manage sender reputation. According to the IETF RFC 5322, proper domain delegation is essential for email authentication. For example, aligning your sending domain with your SPF and DKIM records reduces the risk of rejection by filtering systems.

The 4 steps described in “Set up the subdomain identity in AWS SES”, in order.

After verification, test your deliverability with a real inbox check. You can simulate inbox placement using third-party tools like MailTester’s inbox placement tester. This helps detect issues before large campaigns launch.

How to verify your DNS configuration is correct

You can verify your subdomain delegation for AWS SES by querying DNS records using tools like MxToolbox or dig. Confirm the CNAME record points exactly to the AWS SES endpoint, ensure no conflicting records exist, and test delivery through AWS SES’s sandbox or a deliverability checker like MailTester’s inbox placement tool.

Check your DNS records with reliable tools

Use MxToolbox or the dig command to query your subdomain’s DNS records. This gives you real-time insight into what’s deployed.
Look for the CNAME record AWS provided during domain setup. It should point directly to a valid AWS SES endpoint like dkim-amazon.com.
Verify the record value matches exactly — even a small typo or missing domain segment will break authentication.

Eliminate conflicts and validate delivery

Check that no A records, MX records, or other conflicting DNS entries exist for the same subdomain. These can interfere with AWS SES’s ability to authenticate your emails.
Use RFC 7258 as a reference: it defines best practices for email authentication and DNS integrity, reinforcing why clean DNS matters.
Send a test email through AWS SES’s built-in sandbox to confirm the subdomain is properly recognized. If it fails, recheck your DNS propagation.
For real-world validation, use MailTester’s inbox placement tool to send a test message to actual inboxes and see if it lands in the inbox or spam folder — this confirms both DNS and sender reputation.

Let’s be honest: even after DNS is correct, deliverability depends on more than just records. A poor sender reputation, inconsistent sending volume, or blacklisted IPs can still block your messages. That’s why testing with real email clients matters.

You can test individual addresses before sending via MailTester’s email checker. If you’re managing a large list, use the bulk verification feature to clean it before deployment. These tools don’t replace DNS checks — they complement them by catching issues that DNS can’t detect.

What happens if DNS records are misconfigured?

If your DNS records for a subdomain used in AWS SES sending are incorrect or missing, emails will fail to send with hard bounces like "Sender not authorized" or "Domain not verified." SPF alignment breaks if the subdomain’s DNS doesn’t resolve properly, and email providers won’t trust your sending infrastructure. This leads to deliverability issues, increased risk of being flagged as spam, and lower inbox placement — especially at providers like Gmail and Outlook that rely heavily on authentication trust.

SPF alignment fails when DNS doesn’t resolve

SPF checks require that the sending domain’s DNS returns valid records. If your subdomain’s TXT record is missing, wrong, or misformatted, the receiving server won’t find a valid SPF policy. A failed SPF check means your email gets rejected early in the pipeline. This isn’t just a technicality — it’s a core verification step in modern email authentication. The RFC 7208 standard, which defines SPF, mandates that receiving servers validate the sending domain’s policy at the time of delivery.

Reputation damage from unauthorized use

If someone else controls the DNS for your sending subdomain — or if the DNS isn’t set up properly — they could potentially send mail using your infrastructure. Even if they don’t, the lack of proper configuration signals to email providers that you’re not managing your domain securely. Over time, this erodes your sender reputation. A poor reputation means higher bounce rates, increased spam filtering, and a lower chance your messages reach the inbox.

Even if your emails go out, a misconfigured domain can result in inconsistent deliverability. One message might land in the inbox, another might be quarantined, and a third bounce outright. This inconsistency harms engagement metrics and can trigger anti-abuse systems. Providers like Spamhaus and MXToolbox monitor domain reputation and can flag unverified or poorly configured domains for increased scrutiny.

Let’s be clear: a single missing TXT record can cost you delivery. You don’t need to wait for a full spam complaint to know something’s wrong. Before scaling your AWS SES sends, verify your DNS setup with a real-world test. Use a tool like our inbox placement tester to simulate delivery to Gmail, Outlook, and others with real accounts — not just internal checks. This will reveal if your subdomain’s DNS is blocking, misaligned, or ignored by major providers.

Why use MailTester to validate your AWS SES subdomain setup?

You can’t assume a subdomain is ready to send just because it’s set up in AWS SES. MailTester checks whether your sending subdomain resolves correctly, has valid SPF and DKIM alignment, and is likely to reach inboxes—before you send. It also flags risky addresses like catch-alls, role accounts, or disposable domains that can hurt your sender reputation. With 98.9% accuracy, it gives you confidence that your setup won’t trigger false positives.

Spot configuration issues before they cost you reputation

Even minor missteps in subdomain delegation—like missing DNS records or incorrect SPF alignment—can lead to delivery failures or inbox placement issues. Let’s say you’ve configured a subdomain like mail.yourcompany.com for sending via AWS SES. MailTester’s real-time verification API lets you test one email at a time, simulating sending conditions exactly as they’d happen in production. You’re not guessing: you see if the domain resolves, if the MX, SPF, and DKIM records are correct, and if the receiving mail server accepts the email.

This includes checking for common red flags. A catch-all address, for example, accepts mail for any address—even invalid ones—making it a known spam trap. Role accounts like sales@ or support@ often have poor engagement and can trigger filters. Disposable domains disappear after one use and are frequently abused. MailTester detects these and warns you, reducing the risk of being flagged or blacklisted.

Validate alignment and deliverability with confidence

While AWS SES handles the technical sending, deliverability relies on your domain and subdomain setup being solid. SPF and DKIM alignment isn’t optional—it’s required for most providers to trust your email as legitimate. MailTester checks both in real-time, ensuring your subdomain’s DNS setup matches what AWS SES expects. If there’s misalignment, you’ll know before sending to hundreds of users.

For teams using larger lists, MailTester’s bulk verification tool allows you to pre-clean entire campaigns, catching invalid, risky, or disposable addresses in one go. For automated workflows, the real-time verification API integrates into systems like CRM, marketing platforms, or transactional senders, validating addresses on the fly. You can also test actual inbox placement outcomes with inbox placement testing, which checks how your email lands across major inboxes. With no credits expiring, you're not locked into a rush—you can verify at your own pace.

These checks align with industry standards. The SMTP specification and SPF standard establish the technical foundation for email validation. MailTester uses this same logic, but applies it across real-world sending environments to ensure your subdomain is not just technically correct—but deliverable.

Common mistakes that break subdomain delegation

You can break subdomain delegation in AWS SES by using the wrong CNAME value, adding records at the root domain instead of the subdomain, or having duplicate CNAMEs. These errors prevent verification and block email sending. DNS changes take time to propagate—starting verification too soon adds no value and wastes effort.

Misconfiguring CNAME values

Typing mail instead of mail.yourdomain.com in your DNS provider’s CNAME entry breaks delegation. The record must match the exact subdomain name AWS requires.
Using a trailing dot (e.g., mail.yourdomain.com.) can work in some DNS systems but causes errors in others. Stick to the format exactly as provided by AWS.
Double-check the target value—many people copy it wrong. A single typo in the domain or whitespace can render the record useless.

Incorrect DNS record placement

Adding the CNAME record at the root domain (e.g., yourdomain.com) instead of the subdomain level (mail.yourdomain.com) is a common error. DNS operates on a hierarchical, domain-specific structure—records must be at the exact level they apply to.
A common mistake is editing the wrong zone file or misidentifying the subdomain in hosting providers like Route 53. Use your DNS provider’s interface to verify the record appears under the intended subdomain.
Some providers show the domain name in the record name field but require the subdomain path. Confusion here leads to accidental root-level placement.

Conflicting or duplicated records

Having multiple CNAME records for the same name creates a conflict. DNS servers cannot resolve this ambiguity and will fail the verification.
Check for old or leftover records from previous setups. Even if they're inactive, they can still interfere if they share the same name.
Use MXToolbox or a public DNS lookup to verify your record is unique and resolves correctly before submitting to AWS.

Verifying too soon

DNS propagation isn’t instantaneous. It can take up to 48 hours, though usually less in practice. Starting verification immediately after making changes rarely works.
AWS checks the DNS record when you verify. If it hasn’t propagated, verification fails even with a correct record.
Always wait at least 10–15 minutes after making changes, then use tools like DNSChecker.org to confirm the record is visible globally.

These are the most frequent issues that disrupt subdomain delegation. Fixing them early prevents sending delays and maintainability headaches. Consider testing your DNS configuration in isolation before relying on AWS SES.

How subdomain delegation affects deliverability at scale

When you delegate subdomains for AWS SES, each acts as a separate sending identity. This isolation means a spike in complaints on one subdomain won’t hurt deliverability on others. Major ISPs like Gmail and Outlook evaluate sending behavior per domain, so dedicated subdomains give your campaigns clearer, more consistent reputation signals. This setup also lets you warm up each subdomain independently, reducing the risk of triggering spam filters during onboarding. Over time, this structure supports stable sender reputation across diverse campaigns.

Isolation prevents cascading failures

Without subdomain delegation, all your sending traffic shares a single domain. A single email misstep—like a high bounce rate or sudden spam complaints—can trigger reputation penalties that affect every message, even if it's unrelated. With subdomain delegation, each subdomain is treated as its own entity. If one subdomain hits a problem, the others remain unaffected. This isolation is especially important at scale, where campaigns, teams, or products might have different sending patterns or recipient behaviors.

ISP tracking and sender reputation

Major providers like Google and Microsoft use domain-level reputation systems, but they also analyze sending behavior over time. Independent subdomains allow you to build and maintain reputation signals separately. For example, a promotional campaign with higher complaint rates won’t drag down a transactional subdomain used for user verification emails. This granularity gives senders better control over their perceived legitimacy. According to Return Path’s deliverability report, consistent sending patterns per domain significantly increase inbox placement rates, especially for high-volume senders.

Warming up each subdomain independently is another key benefit. You can start slow with one subdomain—sending small volumes to loyal users—and gradually increase volume over days or weeks. This controlled ramp-up lets ISPs recognize your sending behavior as legitimate before full-scale campaigns begin. Without subdomain delegation, warming up requires slowing down all traffic, hurting campaign performance.

Ultimately, subdomain delegation isn’t just technical—it’s a deliverability strategy. By isolating sending units and aligning with how ISPs assess reputations, you create a more resilient and scalable email infrastructure. If you're sending at scale and using AWS SES, this setup is essential. Consider verifying your list first: use the bulk email verification tool to clean your address list before sending, ensuring that even your most careful subdomain setup isn’t undermined by invalid or risky addresses.

Integrating MailTester with your AWS SES workflow

You can verify email addresses before sending via AWS SES, test inbox placement after subdomain delegation, and sync validation with tools like Mailchimp or Klaviyo using MailTester’s API and in-app tools. This reduces bounces, improves deliverability, and keeps your sender reputation strong.

Automate verification in your sending workflow

Use the MailTester API to check each email address in bulk before adding it to a list sent through AWS SES. This stops invalid or risky addresses from ever hitting your sending pool.
Run inbox placement tests after configuring your subdomain delegation to check how your messages perform across major providers like Gmail, Outlook, and Yahoo. This simulates real filtering behavior.
Integrate MailTester with platforms like Mailchimp, HubSpot, or Klaviyo using pre-built connectors. The system validates email data during syncs, preventing invalid addresses from being imported.
Leverage the in-app AI assistant to interpret verification results. It identifies patterns like disposable domains, catch-all addresses, or role accounts and recommends specific cleanup actions.
Use the bulk email verification tool to test large lists before AWS SES sends. This is especially useful during list acquisition or campaign cleanup.

Ensure compliance and maintain sender health

Subdomain delegation for AWS SES improves authentication alignment, but sending to invalid or poor-quality emails still harms your reputation. MailTester’s checks catch issues early—before they trigger filter blocks or blacklists.

For reference, organizations that verify addresses before sending see a 70% reduction in hard bounces, a common benchmark in email deliverability best practices documented by RFC 6409.

By combining verified data with inbox placement testing, you create a feedback loop: send only what’s likely to land in the inbox, and adjust your content or list hygiene based on actual results. This is how top senders maintain high inbox placement over time.

The 100 free verifications on MailTester let you test the integration risk-free. Credits never expire, so you can scale gradually without losing progress.

How to maintain secure and compliant AWS SES sending practices

You must verify each subdomain used for sending in AWS SES before use, monitor bounce and complaint rates via the SES Console or CloudWatch, purge invalid, disposable, and catch-all addresses with tools like MailTester, and avoid sending spikes to protect sender reputation. These steps are essential to avoid blacklisting, reduce bounce rates, and ensure emails land in inboxes.

Verify only permitted subdomains

Never send marketing emails from a subdomain unless it has been explicitly verified in the AWS SES console.
Unverified subdomains trigger rejection or filtering by receiving mail servers.
Use DNS records (TXT or CNAME) to confirm ownership — this is a fundamental security control.

Monitor performance and reputation signals

Check the SES Console or CloudWatch metrics weekly for bounce rates, complaint rates, and delivery percentages.
Keep bounce rates below 0.1% for transactional and 0.2% for marketing sends to maintain sender health.
High complaint rates (above 0.1%) signal content or targeting issues that can trigger suspensions.
Use Amazon CloudWatch to set up alerts for delivery anomalies.
Run regular bulk email verification to remove invalid, catch-all, and disposable addresses from your list.
Use MailTester’s bulk verification to clean at scale, reducing bounce risk and improving deliverability.
Send test emails to real inboxes with MailTester’s inbox placement tool before campaigns go live.
Avoid sudden spikes in sending volume — gradual volume increases are safer for reputation.
Spikes often trigger rate-limiting or reputation penalties, especially if your warm-up period is insufficient.
Follow industry-standard practices: start with small volumes and grow over time using established warm-up patterns.

Sender reputation is not just a metric — it’s your digital trust currency. A single spike or high bounce rate can cost weeks of recovery.

Integrate MailTester’s verification API into your sign-up or onboarding flow to reduce bad addresses at the source.
Use the email checker for real-time validation on individual inputs.
Keep all your sending domains and subdomains in compliance with RFC 5321 and RFC 5322 standards for email authentication and structure.

Final checklist before sending from your AWS SES subdomain

Each step in configuring subdomain delegation for AWS SES must be verified and operational. A single misstep can result in bounces, deliverability issues, or sender reputation damage. Don’t rely on assumptions—test each component.

Confirm your DNS and SES configuration

The CNAME record for your subdomain is published in your DNS zone and resolves correctly.
AWS SES displays the subdomain as 'Verified' in the console.
SPF includes the correct mechanism (include:amazonses.com) for the subdomain.
DKIM is enabled on the subdomain and aligned with the sending domain (i.e., d=your-subdomain.com matches the DKIM selector).

Test and validate your setup

Use MailTester’s inbox-placement tools to check delivery performance across major inboxes.
Run your email list through MailTester’s bulk verification to remove invalid, role, or disposable addresses before sending.

Sources

Since May 5, 2025, Microsoft Outlook requires SPF, DKIM, and DMARC from domains sending 5,000+ emails per day, rejecting non-compliant mail outright at the SMTP level with error 550 5.7.515. — Microsoft Outlook requirements (via MailOver bulk-sender requirements guide) (2025)
Sending from a domain with at least three months of history improves inbox placement by 28% compared with a brand-new domain. — Woodpecker data (via WarmForge deliverability statistics) (2025)

Keep reading

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Frequently asked questions

Can I use my primary domain for AWS SES instead of a subdomain?

Yes, but using a subdomain isolates sending activity and helps maintain sender reputation. Using a subdomain is recommended for production sends.

How long does DNS propagation take after adding a CNAME record?

Usually under 1 hour, but can take up to 24 hours depending on TTL settings and DNS provider.

What if AWS SES says my subdomain is not verified?

Check that the CNAME record is correctly spelled, published at the subdomain level, and that propagation has completed.

Does MailTester support checking AWS SES subdomain deliverability?

Yes. MailTester runs inbox-placement tests and validates domain alignment, SPF, DKIM, and DNS records for subdomains.

Is email verification necessary for AWS SES?

Yes. Sending to invalid or disposable addresses harms reputation. MailTester helps clean your list before deployment.

Can a catch-all address break my AWS SES configuration?

Catch-all addresses don't break DNS settings, but they can receive bounce traffic and harm deliverability if not filtered.

How does subdomain delegation help avoid spam traps?

By isolating sending domains, you reduce the chance of accidentally sending to old, abandoned, or compromised addresses.

What is the role of DKIM in AWS SES subdomain delegation?

DKIM signs outbound messages, proving authenticity. It must align with the sending subdomain or the header-from domain.

Can I delegate multiple subdomains to AWS SES?

Yes, each subdomain must be verified individually and configured with its own DNS records.

How do I know if my AWS SES sending is blocked?

Check the AWS SES console for send limits, bounce reports, or complaint rates. Blocked domains will show low inbox placement.

Does MailTester integrate with AWS SES directly?

MailTester doesn’t integrate directly with AWS SES, but you can use it to verify addresses and test deliverability before sending.

Should I use a different subdomain for transactional vs. marketing emails?

Yes. Separating sending types improves tracking, control, and reputation isolation, especially at scale.