Preventing Relay-Based Mail Routing in AWS SES

Why does AWS SES routing fail when using shared or open relays?

You're sending transactional emails through AWS SES, and suddenly your inbox placement drops. Bounces increase. You check the logs, run diagnostics, and still can't find the root cause. It’s not the content. It’s not the list quality. The real issue might be hidden in your routing setup.

AWS SES is built to send directly—not to relay mail through external servers. When you route emails through a shared or open relay, you’re treating SES like a conduit for any sender. That breaks the sender reputation model that ISPs enforce. The result? Your mail is flagged as suspicious, throttled, or outright rejected.

Key takeaways

AWS SES must not be used as a relay; it's designed for direct, authenticated sending only.
Open or shared relays accept mail from any sender, making them high-risk and easily abused by spammers.
Routing through untrusted relays violates SPF, DKIM, and DMARC because the origin appears misconfigured or unverified.

How does relay-based routing compromise sender reputation and deliverability?

Relay-based routing in AWS SES creates a shared reputation footprint: when your sending IP accepts mail from any sender, spam from one user can blacklist the entire IP, harming your deliverability. Even if you’re sending legitimate mail, a reputation hit from another sender on the same relay can result in your messages being rejected or marked as spam by filtering systems.

Shared footprints and reputation decay

When a relay accepts mail from unverified or compromised sources, the sending IP accumulates negative signals. This shared risk means one spammer’s actions can taint your messages—even if you use a verified domain and proper authentication. Mail systems treat such IPs as high-risk, especially if they show inconsistent sending patterns or sudden spikes in volume from multiple domains.

Sender reputation is not built on individual messages alone—it’s based on consistent origin, sending behavior, and response rates over time. A relay that routes traffic from many sources fails consistency checks. Filtering systems like those from Return Path or Google’s spam filters observe this volatility as a red flag. A single IP serving diverse, unscreened senders appears less trustworthy, reducing your inbox placement potential.

SPF failures and authentication breakdowns

Even with a verified identity in AWS SES, SPF checks can fail if you route mail through a relay that doesn’t properly authenticate the sender. SPF is designed to validate that the sending server is authorized by the domain’s DNS records. If your relay server isn’t listed in the SPF record for the sending domain, the email will fail SPF checks—regardless of your SES verification status.

For example, if your SES outbound mail is routed through a shared relay that lacks proper SPF alignment, filtering systems may reject it outright. This undermines your deliverability even with technically valid credentials.

Let’s be clear: using a relay-based configuration doesn’t just increase risk—it actively weakens the foundation of email authentication. This makes your messages more likely to be flagged, quarantined, or blocked. You don’t need a relay to send via SES. Properly configured direct delivery, combined with clean address lists, is safer and more reliable.

To avoid these pitfalls, verify your email list before sending—especially if your list includes old or unengaged addresses that might trigger spam traps or invalid routes. Clean lists not only improve deliverability but also help maintain sender reputation, reducing the risk of being caught in a relay-based incident.

Before sending, validate every address in your list. Use real-time tools to catch invalid, disposable, or high-risk emails early.

Verify your entire list upfront with real-time email validation

What’s the role of SPF, DKIM, and DMARC in preventing relay abuse?

You can prevent relay-based mail routing abuse in AWS SES by properly configuring SPF, DKIM, and DMARC. These protocols work together to verify sender identity, reject unauthorized messages, and enforce policies that stop spoofing and abuse. SPF checks sending server legitimacy, DKIM validates message integrity, and DMARC ties them together with enforcement — all vital for maintaining sender reputation and inbox placement.

Let’s walk through how each one stops abuse

Set up SPF records to limit who can send on your domain. SPF defines which mail servers are authorized to send email for your domain. If AWS SES is your only sender, your SPF record must include the AWS SES mechanisms (like include:amazonses.com). Without this, every message from another source fails SPF. This stops spammers from pretending to be you.
Enable DKIM signing on all outgoing messages. DKIM applies a digital signature to each email using a private key hosted by AWS SES. Recipients check this signature with your public key published in DNS. If the signature doesn’t match, the email is rejected. It’s a strong signal that the message hasn’t been tampered with — a key defense against relays that alter content.
Implement DMARC with a policy that enforces SPF/DKIM failures. Without DMARC, even a passed SPF or DKIM check isn’t enough. Recipients often ignore authentication unless a DMARC policy is present. Set your DMARC record to rua=mailto:[email protected] and start with a policy=none to monitor traffic, then move to quarantine or reject once you’re confident. This stops relay abuse by telling receivers how to act when authentication fails.

Why skipping DMARC leaves you exposed

Even with correct SPF and DKIM, abuse can persist if you lack DMARC. Many email providers now treat DMARC as a core signal. According to DMARC.org’s guidance, a domain without DMARC is more likely to be used in spoofing campaigns, even if SPF/DKIM are configured.

You can test your setup with tools like MXToolbox or DMARCian. Always verify that your records are public, correctly formatted, and include your sending sources. For large lists, use verified email checkers to avoid sending to invalid or relay-prone addresses — and reduce abuse risk at the source. Clean your list before sending to ensure only valid, deliverable addresses are included.

How do catch-all addresses and open relays work together in delivery failures?

When a catch-all address is set up, every email sent to any non-existent recipient within that domain is accepted and routed to a single inbox—often without verification. If your AWS SES relay is configured to forward messages to such a domain, it will happily send all emails, including to invalid recipients. Later, when the receiving server rejects the message (because the address doesn’t exist or the server blocks bulk traffic), you get a hard bounce. This creates the illusion the message was valid, but the failure happens after routing, not during validation.

Catch-alls as delivery traps

Catch-all addresses are a common vector for abuse. Spammers exploit them by sending bulk emails to random addresses—since the server accepts everything, the message appears to arrive successfully. But the real recipient never receives it, and the sender is later flagged for high bounce rates or reputation damage. This is especially dangerous when your AWS SES configuration uses an open relay to forward to a catch-all, because the relay treats every email as deliverable, even when it isn't.

Once a catch-all accepts a message, it may be silently discarded or returned with a hard bounce later. This results in failed deliveries that you can’t detect until it’s too late—especially if you’re relying on email tracking or reply monitoring. The problem isn’t the relay itself, but the fact that it’s routing to an address that can’t reliably distinguish valid from invalid recipients.

Preventing misrouting before it starts

Let’s be clear: you can’t fix a catch-all misconfiguration on the receiving end after the fact. The best defense is to detect and remove catch-all addresses before sending. Email list validation tools can identify these addresses during a bulk verification by testing whether the domain accepts emails to non-existent users. They rely on real SMTP and MX checks, not guesswork.

When you verify a list with a service like bulk email list cleaning, you’re checking for more than just syntax—it can detect whether an address would be accepted by a catch-all, whether it’s from a disposable domain, or if it’s a role account (like admin@ or postmaster@). By catching these invalid or risky addresses early, you prevent them from ever reaching your AWS SES relay, reducing bounce rates and protecting your sender reputation.

For example, RFC 5321 and RFC 5322 define how email routing and delivery should work. A true delivery failure should happen early—at the SMTP level—not after the relay has committed to sending. Catch-alls disrupt that model, allowing invalid messages to proceed through the system. This breaks the delivery feedback loop and harms deliverability.

So the real fix isn’t in adjusting your relay configuration. It’s in cleaning your list before any mail is sent. Tools that check against real MX records, test for open relays, and detect catch-alls help you stay compliant, reduce bounces, and avoid blacklisting. You can't control the remote server’s behavior—but you can ensure your own sends are targeted, valid, and trusted.

What happens when you send from an SES identity that doesn’t match your SPF record?

If your SPF record doesn’t include Amazon’s SES servers, the receiving mail server will see your message as failing SPF authentication. Even if your email content is clean and your sending reputation is strong, this failure often results in delivery failure or spam filtering. This is especially harmful when using SES as a relay—there’s no consistent alignment between the actual sending server and the domain in the FROM address.

SPF failure means your messages are treated as suspicious

When a recipient server checks your SPF record, it verifies whether the sending IP (in this case, an AWS SES server) is authorized to send on behalf of your domain. If SES isn’t listed, the check fails. Major providers like Google and Microsoft do not allow SPF fails to pass through their filters, even for low-volume or one-off messages.

Think of SPF as a digital handshake: if the sender doesn’t have the right credentials, the door stays shut. You might believe you’re following best practices, but the absence of a valid SPF alignment is enough to trigger spam score penalties.

Relay use amplifies the risk of misconfiguration

When you send from SES as a relay—especially via applications or services that don’t handle authentication correctly—you create a mismatch. The server physically sending the email (SES) is not your domain’s origin, but the email claims to be from your domain. Without proper SPF alignment, that claim is invalid.

This is common in automated workflows, such as sending transactional emails from a third-party app via SES. The app may send from a domain like [email protected], but if that domain’s SPF doesn’t include SES, the message fails even if everything else is correct.

According to RFC 7208 (the current SPF standard), SPF failures are not just ignored—they are explicitly used as a signal for email rejection or high-risk classification. The SPF check is a foundational part of modern email authentication, and when it breaks, delivery suffers.

Always verify that your domain’s SPF record includes include:amazonses.com. Multiple includes can exceed the DNS lookup limit (10), so use mechanisms like include:amazonses.com sparingly and combine with all policies carefully.

For teams sending hundreds or thousands of messages via SES, pre-checking your sender identities is critical. A single misalignment can result in a 20% or higher rejection rate across large campaigns.

Use verified sender domains, and ensure your SPF record explicitly authorizes AWS SES. You can validate your setup with tools like MxToolbox or Spamhaus to check your SPF record’s scope and alignment.

Don’t assume your setup is safe just because SES works sometimes. Automated checks and consistent validation prevent silent delivery failures. For bulk list hygiene, you can also verify lists before sending to reduce bounce and rejection rates. Learn more about cleaning your list at how to clean your email list.

How to verify email addresses before sending through AWS SES to prevent relay abuse?

You can prevent relay-based misconfigurations in AWS SES by validating every email address in your list before sending. Use a real-time verification API to check each address for validity, catch-all status, or disposable domain usage. A 98.9% accurate system identifies and blocks invalid or high-risk addresses—reducing bounces, protecting sender reputation, and minimizing the risk of your AWS SES account being flagged for abuse.

Pre-send validation is mandatory for safe AWS SES operation

Use a real-time verification API to test each email address before it enters your AWS SES sending queue. This stops invalid or malicious addresses from ever being transmitted.
Filter out disposable email domains (like tempmail.org or mailinator.com) that are commonly used for abuse and often have high bounce rates.
Remove catch-all email addresses—those that accept all incoming mail regardless of recipient—as they can be hijacked by spammers to exploit your SES configuration.
Block addresses that are flagged as invalid or syntactically malformed, as they can trigger bounce loops and harm your sender reputation.
Verify that the domain has valid DNS records (SPF, DKIM, DMARC) through automated checking, since misconfigured domains are a common relay abuse vector.

Why catching abuse early matters

Relay abuse occurs when spammers exploit poorly validated sending systems to route messages through your AWS SES account. This leads to blacklisting, reputation damage, and potential account suspension. A 98.9% accurate verification process—such as the one used by Email List Validation—catches 989 of every 1,000 invalid or risky emails before they are sent.

According to the IETF’s RFC 6598, using non-routable addresses or misconfigured mail relay systems can lead to unintended forwarding of spam. Preventing this starts with address validation at the edge.

By integrating a verified list into your AWS SES workflow, you reduce the chance of being exploited—especially when sending to large lists. This is not optional. It’s an operational requirement.

For a full bulk cleanup, consider tools like bulk email list cleaning before you send. Or, for high-volume systems, connect the real-time verification API directly into your send pipeline to validate addresses on-the-fly.

You can prevent relay-based mail routing issues in AWS SES by validating your email lists before sending. Tools that check for disposable domains, role accounts, invalid syntax, and catch-all addresses help identify configurations that could be abused or misused, reducing the risk of your sender reputation being damaged by unexpected bounces or spam traps.

Disposable domains and role accounts are red flags for relay abuse

Disposable email domains (like mailinator.com) and role accounts (like admin@ or info@) are often used to circumvent spam filters or exploit open relays. Sending to them increases the chance of your messages being flagged as suspicious or rejected. Bulk verification tools catch these early by checking against known lists of disposable domains and role-based patterns.

For example, role accounts like postmaster@ or abuse@ are commonly set up to accept mail from any sender—ideal for relay abuse. When your AWS SES sends to them, it may appear as if you’re part of a spam network, even if you're not. Verifying these addresses upfront prevents unintended exposure and helps maintain trust with email providers.

Catch-alls expose poor list hygiene—validation blocks them before delivery

Catch-all email addresses accept all incoming messages, regardless of recipient validity. They’re a common misconfiguration in some domains and a known attractor for spam. If your list contains catch-alls, your emails will still “send,” but they’ll go to a mailbox that collects everything—often leading to high bounce rates or poor sender reputation.

Before you send through AWS SES, bulk verification identifies catch-alls by analyzing the domain’s mail server response. This lets you remove or flag these addresses so they don’t trigger hard bounces in production. This is critical for preventing your domain from being flagged for abuse by services like Spamhaus or MXToolbox, which monitor sending patterns and reputation signals.

Let’s be honest: if your list has even a few of these, your deliverability takes a hit. The fix isn’t complex—it’s just discipline. Using a service like bulk email list cleaning lets you catch these issues before sending, reducing bounce rates and protecting your sender reputation.

What’s the difference between sending directly via AWS SES and relaying through a third-party system?

Direct sending via AWS SES routes your emails through Amazon’s authenticated infrastructure, ensuring SPF alignment, consistent sender reputation, and lower risk of rejection. Relaying through third-party systems introduces unverified intermediaries, breaks SPF alignment, and increases exposure to blacklisting and spoofing—especially if the relay server lacks strong authentication or proper IP reputation tracking.

Here’s how it breaks down in practice:

Send directly from AWS SES using your verified domain. This ensures SPF records are properly aligned and your sending server is pre-authenticated. AWS manages infrastructure-level safeguards, including rate limiting, spam detection, and IP reputation management.
Validate your domain’s SPF, DKIM, and DMARC records. When sending natively via SES, your domain’s DNS records must align with AWS’s servers. Misalignment here causes deliverability failures even if the email is otherwise valid. Use tools like MXToolbox to test DNS setup before scaling.
Avoid using third-party relay services unless strictly necessary. Relay systems treat your email as incoming traffic, breaking the path of authentication. The relay server becomes the origin IP, not your AWS SES instance—so your SPF alignment vanishes.
Understand the implications of unverified relays. If your relay server uses a shared IP or lacks proper authentication, it may be listed on blocklists like Spamhaus. This harms your reputation even if you’re not the source. RFC 7208 (SPF) defines strict rules for sender validation—relays often violate these by design.
Test deliverability before sending at scale. Even with correct configuration, some messages may still land in spam or fail outright due to reputation or content issues. Use inbox placement testing to verify your message reaches real inboxes. Test real inbox delivery with tools that simulate how major email providers treat your content.

Why misconfigured relays lead to delivery breakdowns

When you relay through a third-party system, you lose control over the sending environment. Amazon’s reputation management, IP warming, and real-time threat detection don’t apply. The relay might be on a blacklisted IP. It might not enforce DKIM signing. It might send too many emails too quickly. All of this triggers spam filters and increases your risk of being flagged as a source of abuse.

Relay-based routing is a common vector for spoofing and abuse, especially when domains aren’t properly aligned. Direct SES sending prevents these risks by keeping the entire path under a single, trusted, authenticated system.

Before sending high-volume campaigns, verify your list hygiene—invalid addresses and catch-all domains inflate bounce rates and hurt sender reputation. Clean your list before sending to reduce failures and protect your standing with providers.

How to use inbox placement testing in concert with email validation

You can't rely on valid email addresses alone to guarantee inbox delivery, especially when using AWS SES with relay-like routing during migration. Even correctly formatted addresses may end up in spam folders or get blocked if reputation or content triggers are present. Pair inbox placement testing with pre-send validation to catch deliverability issues before they affect your sender reputation. This dual layer ensures your messages land in real inboxes, not just bounce or get filtered.

Validate before you send

Use email validation to clean your list before sending—remove malformed, non-existent, and disposable addresses. This cuts bounce rates and protects your sender reputation.
Run bulk verification via an API or dashboard to catch invalid or risky addresses at scale. Tools like Email List Validation’s bulk verification identify issues like catch-all domains and role accounts that may not bounce immediately but still hurt deliverability.
Check for common misconfigurations: ensure your SES domain or identity is properly authenticated via SPF, DKIM, and DMARC. Misconfigurations can silently degrade inbox placement, even if the address is valid.

Test delivery to real inboxes

Use inbox placement testing tools to send real messages through your AWS SES setup to actual inbox providers (Gmail, Outlook, Apple Mail). This confirms your messages aren't being flagged or blocked during transit.
Run tests from both your production and temporary relay configurations. Even short-term relay setups can expose your domain to reputation risks if messages are marked as spam.
Test different content variations—subject lines, body formatting, and links—to see how inboxes react. Some content triggers are known to trigger spam filters even with proper authentication. Spamhaus maintains real-time DNS-based blocklists that reflect current spam patterns.
Compare results across providers. A message might land in Gmail’s inbox but end up in Outlook’s junk folder. Use this to refine your message, branding, and authentication.
Combine with inbox placement testing to get a full picture of your message's journey. This step is essential when transitioning or using relay patterns, as it detects issues that validation alone will miss.

Let’s be clear: validation finds bad addresses. Inbox placement tests show you if good ones still fail delivery. The combination matters—especially in dynamic AWS SES environments where relay configurations may mask underlying issues until they’re too late.

Can you use AWS SES with third-party tools without creating relay risks?

Yes — you can safely use AWS SES with tools like Mailchimp, Klaviyo, or HubSpot as long as they send directly to the SES API and don’t act as relays that accept arbitrary sender domains. If the tool forwards email through a shared server that accepts mail from any sender, it creates a relay risk. That’s not how direct SES integrations work, so they’re generally safe when properly configured.

How third-party tools integrate with AWS SES safely

When Mailchimp, Klaviyo, or HubSpot integrate with AWS SES, they do so via the SES API directly. They don’t receive mail from arbitrary domains and then forward it through a relay server. Instead, they authenticate using your AWS credentials and send individual messages directly to SES’s endpoints. There’s no middleman accepting unsolicited mail, so relay-based misconfigurations don’t apply.

What matters is whether the third-party is acting as an outbound sender or a relay. If it uses SES as a delivery engine and doesn’t accept inbound mail from untrusted sources, it’s not a relay. The sender domain is always controlled by the app or user, not by an open forwarding system. This is a key distinction in deliverability best practices.

What to check before trusting an integration

Not all integrations are created equal. You must verify that the tool doesn’t forward mail through an intermediary server that could accept any sender address. Some older or poorly designed setups have been known to use shared relays — especially in shared hosting environments — which can trigger spam filters or violate AWS terms.

Always check that the integration uses AWS SES’s secure SMTP or API endpoints directly. Look for documentation that says “sends directly to SES” or “uses AWS IAM for authentication.” Open relays are rare today, but they still show up in tools that haven’t properly implemented authentication or that rely on legacy forwarding systems.

One way to reduce risk is to validate your email list before sending. A clean list reduces the chance of hitting a relay filter, even if a misconfiguration exists on the tool side. You can run a bulk verification to catch invalid, disposable, or risky addresses before sending via SES.

Run a bulk verification on your list to ensure your send rates stay high and your reputation stays intact.

The SMTP standard explicitly warns against open relays. If your third-party tool is not a relay, you’re following RFCs and AWS policies correctly.

Final takeaway: Prevent relay issues by sending only to verified, valid addresses

A misconfigured relay exposes your AWS SES setup to abuse, regardless of your reputation or technical controls. It’s not a minor misstep—it’s a systemic weakness that can lead to blacklisting, suspension, or even takeover.

Email validation is the most effective way to prevent this. By filtering out invalid, disposable, or abusive email addresses before they enter your send queue, you eliminate the risk of relay misuse at the source.

Build a resilient setup

Use Email List Validation to verify addresses in bulk or via API.
Send directly through AWS SES—no relays, no third-party routing.
Enforce SPF alignment and DMARC policies to ensure sender identity integrity.

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Ready to put this into practice? Email List Validation verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

What is a relay-based mail routing misconfiguration in AWS SES?

It occurs when your sending service forwards mail through a shared or external server that accepts mail from any sender. This violates AWS SES's direct-sender model and damages sender reputation.

Can AWS SES be used as a relay for outgoing mail?

No. AWS SES is designed for direct sending only. Using it as a relay exposes your domain to authentication failures and increases the risk of blacklistings.

What happens if SPF fails on AWS SES messages?

Receiving servers flag the email as suspicious or reject it entirely. This leads to hard bounces, reduced inbox placement, and a damaged sender reputation.

How does email validation prevent relay abuse?

By identifying catch-all addresses, disposable domains, and invalid syntax, validation ensures that only real, legitimate recipients receive your messages—reducing the chance of relay-like behavior.

Why do catch-all addresses cause delivery issues?

They accept all mail regardless of recipient. This makes them a common target for spammers and often leads to high bounce rates and spam complaints.

What’s the role of DKIM in preventing relay issues?

DKIM signs each message with a private key. If the signing server is not authorized, the signature fails—even if the message is otherwise valid.

How does DMARC protect against relay-based spoofing?

DMARC sets policies that tell receiving servers what to do with emails failing SPF or DKIM. Without it, failed auth checks may be ignored, increasing spoofing risk.

What’s the best way to test deliverability after fixing a relay misconfiguration?

Use inbox placement testing with real inboxes to assess whether emails now reach inboxes and avoid being marked as spam.

Does using a third-party email tool with AWS SES create relay risks?

Only if the tool acts as a relay. Direct integrations with SendGrid, Mailchimp, or HubSpot via SES API are safe if they do not forward mail through shared servers.

How often should you validate email lists before sending?

Before every major send. For high-volume campaigns, validate in real-time or use bulk verification on the full list monthly to maintain list hygiene.

Can a 98.9% accurate email validation system catch all problematic addresses?

It catches 989 out of every 1,000 invalid or risky addresses. It cannot catch every edge case, but it reduces risk significantly and is the most effective pre-send filter available.

Why is it important to align SPF with AWS SES?

SPF defines which servers can send on your domain’s behalf. If SES isn’t included in the SPF record, mail fails authentication—even with valid content.