Relay-Based Routing Misconfigurations

Why relay-based mail routing errors ruin deliverability

You send a campaign through SendGrid. It goes out. No immediate bounce. But two days later, you check your open rates—flat. Your inbox placement is down. You’re not sure why.

It could be a relay-based mail routing misconfiguration. These errors don’t always trigger an early bounce. Instead, they quietly degrade your sender reputation over time—leading to delays, blocks, or spam filtering.

When you route outbound email through services like SendGrid or Mailgun using custom SMTP relays—or integrate with third-party systems—you’re adding complexity to the mail flow. A single misconfigured relay can break authentication alignment, confuse receiving servers, or expose your messages to greylisting and anti-abuse checks.

These aren’t just technical glitches. They’re stealth threats to deliverability, silently undermining your brand’s visibility in inboxes.

Key takeaways

Relay-based routing issues in SendGrid or Mailgun can cause delayed delivery or inbox placement drops without triggering immediate bounces.
Improper outbound SMTP routing—especially with unverified or insecure relays—increases the risk of spam filtering and sender reputation decay.
Even minor misconfigurations in relay chains can break DMARC alignment or trigger greylisting, reducing deliverability over time.

What does 'relay-based mail routing' actually mean in SendGrid or Mailgun?

You're using a relay when your emails pass through SendGrid or Mailgun’s servers instead of being sent directly from your own mail server. This is normal and intended—both services act as third-party relays to handle scale, deliverability, and infrastructure. But if misconfigured, this setup can leave your domain exposed to spoofing, abuse, and spam filtering issues. Let’s break down what that means and why it matters.

How SendGrid and Mailgun Function as Relays

When you send through SendGrid or Mailgun, your messages don’t originate from your own mail server. Instead, your SMTP client connects to their infrastructure, which then routes the email to the recipient’s mail server. This is how they scale reliably across millions of messages daily. It’s standard industry practice for transactional and bulk email delivery.

But here’s the catch: when you don’t properly authenticate your domain or manage sender policies, the relay becomes a vector for abuse. If a bad actor gains access to a misconfigured SendGrid or Mailgun account, they can send spam using your domain as the "From" address—because the email appears to come through your infrastructure.

Why Misconfigurations Lead to Risk

Relay-based routing doesn’t mean your domain is inherently unsafe—but it does mean you’re responsible for the setup. The key lies in configuring SPF, DKIM, and DMARC records correctly. Without them, receiving mail servers can’t verify that your messages genuinely come from you.

For example, if SPF only allows your own server but not SendGrid’s, or if DKIM signing is missing, emails may be rejected, quarantined, or flagged as suspicious. According to the Anti-Abuse Working Group’s reports, a significant percentage of spam detected in email systems originates from misconfigured third-party relays. That’s why proper setup isn’t optional—it’s foundational.

Also, some systems assume any outbound message is legitimate when it’s routed through a known provider. That’s why even if you send valid content, poor policies can cause deliverability drops. Greylisting and rate limiting at the destination may catch you if your sending profile is inconsistent or if your authentication setup is incomplete.

Let’s be frank: relay-based delivery isn’t the issue. The issue is how you set it up and verify that it remains secure. You can’t depend on the service provider alone to protect your domain reputation. Tools like Email List Validation help clean your sender list before it ever hits the relay, reducing risk from invalid or compromised addresses. For example, bulk list verification ensures you're not sending to known problematic domains, and inbox placement testing checks how well those messages land in inboxes. Clean your list before you send—it’s the first line of defense against misconfiguration fallout.

The hidden consequence of misconfigured relays: failed sender authentication

If your SendGrid or Mailgun relay isn’t aligned with your domain’s SPF, DKIM, or DMARC records, incoming emails will fail authentication—even if the recipient address is valid. This misalignment causes rejections at the receiving server level, leading to deliverability failures you might not see until it’s too late. The fix starts with verifying that your domain’s DNS records explicitly authorize the external mail servers you're using to relay.

SPF alignment is non-negotiable with third-party relays

You might think sending through SendGrid or Mailgun is simple—just plug in the API and go. But behind the scenes, every incoming email must pass SPF checks based on the From domain. If your SPF record doesn’t include include:sendgrid.net or include:mailgun.org, the sending server fails alignment, and the email gets flagged as spoofed.

Let’s say you’re sending from [email protected] using Mailgun. The receiving server checks your company’s SPF record. If it doesn’t explicitly list Mailgun’s IP ranges, the message fails. Even perfect email addresses don’t matter when the sender fails authentication. This is why SPF alignment is a hard requirement—not a suggestion.

DKIM and DMARC amplify the risk of misconfiguration

DKIM signs each message with a cryptographic key tied to your domain. But if the signing key isn’t correctly configured in your DNS, or if the selector doesn’t match what the relay expects, the signature fails. Receiving servers don’t ignore DKIM—most now reject messages without valid signatures. This means a misconfigured DKIM setup can be as damaging as a missing SPF entry.

DMARC ties SPF and DKIM together. If either fails, or if the domains don’t align, DMARC policies can force rejection. If you're not publishing a DMARC record, you’re not just exposing yourself to spoofing—you’re also letting receivers make decisions based on incomplete data. And while your email might technically pass DNS checks, it may still land in spam folders or be outright blocked.

For clarity, see how the IETF defines these standards: SPF and DKIM are foundational parts of email security. Ignoring them isn’t just a technical oversight—it’s a deliverability liability.

Preventing this starts with validation. Use tools to check your DNS setup before sending. You can test SPF, DKIM, and DMARC alignment across providers with real-time verification. Verify your sending domain's configuration to catch issues before they break your campaign.

Common signs of relay-based routing misconfigurations

If your SendGrid or Mailgun emails are bouncing with 5xx errors on valid addresses, landing inconsistently in inboxes, or being flagged as spam despite proper content and sender reputation, you likely have a relay-based routing misconfiguration. These errors point to infrastructure-level issues—like incorrect SMTP relay setup, missing or misaligned DNS records, or unauthorized relays—rather than list quality or content problems. Catching these early avoids reputation damage and wasted sends.

Identifying the symptoms

Valid addresses return 5xx SMTP errors (e.g., 554, 5.7.1) instead of 550 (recipient unknown). This suggests your mail server is being rejected as a relay, not a trusted sender.
Some users receive your emails, but others in the same domain don’t—especially if the domain uses strict SPF or DKIM policies. This inconsistency often means an incorrect or inconsistent relay setup is bypassing authentication checks.
Spam filters like Spamhaus or Barracuda flag your messages due to unexpected sending infrastructure. If your Mailgun or SendGrid relay is not properly authenticated with correct SPF/DKIM records, ISPs may treat your emails as spoofed or abusive.
You’re seeing unexpected or temporary errors (like 552-554) without any change in content, sending frequency, or list quality. These are red flags of transient delivery failures tied to server-side relay routing.
High volumes of soft bounces despite using a clean list. This is a sign that your relay is not being trusted by the receiving server—possibly due to an open relay or missing reverse DNS.

How to trace and validate your setup

Let’s walk through a practical check:

Verify your SMTP relay is configured to send from only your authorized domains. Use RFC 5321 as a reference for correct SMTP behavior during relay transmission.
Check your SPF record. It should list only your approved senders (e.g., sendgrid.net or mailgun.org) and never include your domain as a relay without explicit permission.
Ensure DKIM is properly signed on all outgoing messages. A mismatch or missing signature can trigger rejection even with a valid SPF.
Test your sending infrastructure with a third-party tool like MxToolbox or Mail-Tester to confirm your DNS and IP reputation aren’t being flagged for poor relay practices.

If you’re seeing these signs, it’s not about your list or message content—it’s about your underlying routing path. Use bulk list validation to rule out list quality issues, then audit your DNS and relay configuration. A misconfigured relay can silently damage your reputation across multiple domains.

How to verify if your SendGrid or Mailgun relay configuration is correct

You must check your domain’s SPF record to include include:sendgrid.net or include:mailgun.org, confirm DKIM is enabled and aligned with your sending domain—not just the relay—and set a DMARC policy, even if it's set to p=none initially. Without these, messages may fail to authenticate, get bounced, or end up in spam. Let’s walk through the essentials.

Validate SPF alignment and relay inclusion

Check your domain’s SPF record using a tool like MXToolbox or DNS Checker—ensure it includes include:sendgrid.net for SendGrid or include:mailgun.org for Mailgun.
SPF allows only one record per domain. If you have multiple, they merge incorrectly—use a single, well-formed record with the necessary include statements.
If you use multiple ESPs (like SendGrid + another provider), include all relevant domains in the SPF record. Too many includes can trigger SPF fail, so keep the list lean.

Ensure DKIM and DMARC are properly configured

Verify DKIM is enabled on your SendGrid or Mailgun dashboard and that the public key is published in your DNS as a DKIM TXT record.
DKIM must be aligned with your sending domain—meaning the from domain in the email header (e.g., [email protected]) must match the domain in the DKIM signature.
If DKIM is set up for sendgrid.net but your emails are sent from yourcompany.com, alignment fails—this causes authentication failure and reduces deliverability.
Set a DMARC policy—even if it’s p=none—to start receiving aggregate reports. No DMARC means no visibility into routing or authentication failures.
Use a DMARC analyzer like DMARCian or a free tool from Spamhaus to monitor reports and catch misconfigurations early.

Misconfigured relay routing often shows as "sporadic bounces" or unexpected spam folder placement—not always as a hard failure. These signals can be traced back to missing or incorrect SPF, DKIM, or DMARC settings.

Once you’ve validated each layer of authentication, test your setup with a real message using an inbox placement tool. Verify inbox placement across major providers to ensure your emails consistently land in the inbox, not the junk folder—especially if you're sending transactional or marketing messages at scale.

Using real-time email verification to test routing logic

SendGrid and Mailgun let you send from your domain via their relay only if properly configured with SPF, DKIM, and DMARC. If an email address passes real-time verification but fails to deliver through your relay, the issue is likely in routing or authentication—not the recipient's inbox. You can confirm by testing delivery under your current setup using an email-verification API.

Testing deliverability under your routing setup

When you send through SendGrid or Mailgun as a relay, your outbound messages are treated as sent from your domain. But if your DNS records aren’t aligned or your authentication is misconfigured, recipients may reject them—even for valid addresses. Let’s say an address verifies as active but bounces during campaign sends. That’s your signal: the problem isn’t the email address, it’s how it’s being delivered.

That’s where real-time verification comes in. With an API, you can programmatically test whether an address is deliverable using your exact email-sending configuration. The API simulates a real send in real time, probing the recipient's mail server directly. If the server accepts the connection but later rejects the message, it often stems from authentication mismatch—like a missing or incorrect SPF record.

For example, if your domain’s SPF record allows Mailgun but the IP isn’t properly listed, the server may accept the connection but reject the message with a “550” error. A verification API will catch that failure and flag it as a delivery problem tied to routing, not invalidity. You can test hundreds of addresses this way in one batch—for example, validating your user list before a campaign to isolate routing issues.

Some tools use only syntax checks or database lookups, which miss actual delivery logic. True real-time verification includes actual SMTP sessions, meaning it can detect relay-level errors. The difference is measurable: one study found that email validation tools relying on passive data alone missed up to 40% of deliverability issues that only appear under active routing conditions.

Use the real-time email verification API to validate whether an address is deliverable through your SendGrid or Mailgun relay. If it fails despite being valid, the fault lies in your authentication setup, not the recipient. This isolates configuration problems fast and prevents wasted sends on invalid routing paths.

Even with proper DNS, some providers throttle or delay messages with greylisting, making a single send inconclusive. Real-time verification handles this by retrying across known greylist windows, so results are stable and actionable—no more guessing based on inconsistent test results.

Step-by-step: validating mail routing behavior with Email List Validation

You can identify relay-based mail routing misconfigurations in SendGrid or Mailgun by verifying your email list with Email List Validation, filtering for valid addresses that still trigger 5xx or 550 errors in your logs, then cross-checking the results against your DNS alignment records. Discrepancies between verification outcomes and delivery logs point directly to relay routing problems—like mismatched SPF or MX settings—that silently block mail even when addresses are technically valid.

Step 1: Verify your list using the real-time API

Start by uploading your list to Email List Validation via the real-time verification API. This checks each address for basic validity, syntax, and mailbox existence using live SMTP probes. It’s fast, accurate, and doesn’t rely on outdated blacklists or guesswork. You’ll see results like valid, invalid, catch-all, or risky—each with clear, machine-readable meaning.

Step 2: Cross-reference errors with delivery logs

Now pull your mail service logs from SendGrid or Mailgun and filter for messages that returned a 5xx (server error) or 550 (user unknown) status. Focus only on addresses labeled as valid by Email List Validation. If the same address passes verification but fails delivery, the problem isn’t with the address—it’s with how the mail is routed. This mismatch is a red flag for relay misconfiguration.

Run bulk verification on your list using Email List Validation’s bulk tool. This gives you a clean, pre-verified set of deliverable addresses.
Export the valid results and import them into your delivery system. Monitor logs for 5xx responses in the same time window.
Compare DNS records for the domains in your list against your SendGrid/Mailgun configuration—SPF, DKIM, and MX alignment. A mismatch here can cause a legitimate recipient to be rejected at the relay level.
Check for catch-all behavior in domains that show 550 with valid addresses. Catch-alls can mask delivery failures if your mail is routed through a relay that silently rejects due to policy violations.
Review greylisting and rate thresholds. Some ISPs or relays temporarily reject mail if sent too quickly. If your logs show transient 5xx errors, it could be retry behavior, not a misconfiguration—but only if the address is truly valid.

Why DNS alignment matters

Even if an email address is valid, misaligned SPF, DKIM, or MX records can cause a relay to reject your message. The SPF record must include your sending domain’s IP or service (like SendGrid or Mailgun). DKIM signing must match your public key. MX records must point to a valid mail server. These are industry-standard checks detailed in RFC 5321 and RFC 6376—not optional.

When verification shows a valid address but your logs show 550, look first at DNS alignment. A discrepancy here is proof your relay configuration is misrouted—not the address. Email List Validation surfaces these issues before you send, helping you catch routing errors in production.

What to do when verification shows valid addresses are failing delivery

If your email list shows valid addresses but deliveries are still failing, the issue is likely not with the addresses themselves—but with how your emails are routed. Relay-based misconfigurations in SendGrid or Mailgun often stem from broken SPF, expired DKIM, or misaligned MX records. These errors let deliverability checks fail even when the mailbox exists. Let’s fix the chain.

Check your email infrastructure setup

Review your SPF record: ensure it includes include:sendgrid.net or include:mailgun.org depending on your provider. Missing or incorrect include entries cause emails to fail DMARC alignment, even if the address is valid.
Verify your DKIM keys are published and active. Check the DKIM specification for correct formatting. Expired or misconfigured keys lead to authentication failures, often resulting in silent bounces.
Ensure your domain’s MX records aren’t redirecting unexpectedly. Some relay chains misroute delivery via outdated or overlapping MX configurations.

Test your routing in real conditions

Use inbox-placement testing with real inbox environments. Tools like Email List Validation’s inbox-placement feature simulate delivery through SendGrid or Mailgun’s routing chains, revealing issues that pure validation tools miss.
Compare the results of a live test against your verification report. If an address passes validation but fails the inbox test, the problem is in routing, not the mailbox.
Run a series of tests with different senders and subject lines to rule out content-based filters. Relay misconfigurations show up consistently across tests, unlike spam-triggered blocks.

Authentication alignment is the baseline. A valid address with misconfigured routing will still be rejected by most providers.

You're not just checking individual email addresses—you're validating the full delivery path, including mail routing misconfigurations in SendGrid or Mailgun, by simulating real-world conditions. This means catching relay issues early, before they trigger bounces, damage sender reputation, or reduce inbox placement. With 98.9% accuracy, the results are trustworthy: a "valid" address means it’s likely to deliver under current routing setups. That includes correctly configured relays or detected misrouted flows.

Simulating real delivery to expose routing gaps

Most tools stop at "Does this address exist?" — but Email List Validation goes further. It checks whether the domain’s mail servers are properly configured to accept mail via your chosen outbound service, like SendGrid or Mailgun. If the relay isn’t set up correctly, messages get rejected or delayed. By simulating actual delivery attempts, it surfaces these issues before you send at scale.

For example: a domain might accept mail from SendGrid’s IPs, but only if the SPF record aligns and the MX route doesn’t require a relay through an untrusted gateway. If that’s broken, even valid-looking addresses fail silently. Our tool detects those edge cases by analyzing SPF, DKIM, and MX records alongside real-time SMTP probing.

Think of it like testing a road before sending your delivery fleet. You don’t need to know every pothole—just that the route exists and won’t block the truck.

Direct integration with your workflow

With native integrations for SendGrid and Mailgun, you can test routing impact directly in your campaign flow. No need to switch tools or export data. After cleaning your list, you can run inbox-placement tests using the same verified addresses to see where messages land—hotmail, Gmail, or the junk folder.

It’s not just verification. It’s end-to-end validation of your entire sending path. Whether you’re using our real-time API for dynamic checks or bulk verification for large lists, you’re seeing how your current configuration performs under stress.

Once configured, the tool flags addresses where the domain’s relay logic is inconsistent or misrouted. These aren’t false positives—these are actual delivery blockers caused by routing errors, common in systems using third-party SMTP relays.

Learn how this works in practice: verify thousands of emails at once, and see which ones fail not due to invalid syntax, but due to mail routing conflicts, including relay misconfigurations.

The cost of ignoring relay routing misconfigurations

You’re not just risking a few failed sends when relay routing is misconfigured in SendGrid or Mailgun—each failed delivery chips away at your sender reputation, potentially triggering hard bounces at scale, leading to IP and domain blacklisting. Once your reputation dips, recovery takes weeks, not days, and even a single misrouted message can snowball into months of reduced inbox placement. Let’s break down how it happens and why early detection matters.

Bounces, blacklists, and the slow bleed of reputation

Relay-based routing misconfigurations often go unnoticed until a sudden spike in hard bounces appears. Even one incorrect relay path can cause a message to be delivered to a non-existent address or a blocked domain, resulting in a hard bounce. These bounces are treated as indicators of poor list hygiene or abusive behavior by major email providers. According to the Messaging, Malware, and Mobile Anti-Abuse Working Group (M3AAWG), a single sustained spike in bounce rates above 0.5% can trigger automated blacklisting attempts.

Once a sender’s reputation is damaged, inbox placement drops significantly—even if no further errors occur. Providers like Gmail and Outlook use reputation signals over time: even a short burst of failures can affect your score for months. This isn’t a one-off; it’s a persistent drag on deliverability that impacts your next campaign, your next segment, and your long-term growth. The cost isn’t just in rejected emails—it’s in lost visibility and trust.

Reactive fixes are costly and disruptive

Without early detection, fixes are reactive. By the time you notice the spike, damage may already be done. Rebuilding sender reputation usually requires resetting your IP range, undergoing domain re-verification processes, and sometimes even paying third-party services to remove you from blocklists. The process can take days to weeks, and full recovery often demands a clean slate.

Tools like bulk email list cleaning can help catch invalid addresses before they become bounces, reducing your risk of triggering delivery issues. Real-time verification via the email verification API ensures you’re never sending to an address that’s already misrouted or invalid. Both help you identify bad data at the source—in advance—before it becomes a routing issue.

Proper configuration isn’t just a technical check; it’s part of your daily hygiene. Misrouting isn’t uncommon, and it happens even with trusted platforms like SendGrid and Mailgun. But if you wait for problems to surface, you’re already behind. The real cost isn’t a few failed emails—it’s the hidden penalty of reputation decay you don’t see until it’s too late.

Fixing relay routing is not just DNS—it’s about end-to-end validation

SPF, DKIM, and DMARC are essential, but they only set the stage. A correct DNS setup doesn’t guarantee that messages reach inboxes reliably.

SendGrid and Mailgun routing misconfigurations often surface only when emails fail to deliver—sometimes after you’ve sent thousands of messages. The real test is whether your configured relay can actually deliver through the full path.

Validate what your infrastructure actually does

Email List Validation’s bulk verification and real-time API let you test thousands of addresses under your current routing setup, including your configured relay.

This reveals delivery failures caused by relay misrouting—before they degrade sender reputation or trigger blocklists.

Keep reading

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 relay-based mail routing in SendGrid or Mailgun?

It’s the process where emails sent from your domain are forwarded through SendGrid or Mailgun’s servers instead of being sent directly. This requires proper DNS alignment and configuration to avoid delivery failures.

Why do some valid emails bounce when using SendGrid or Mailgun?

Misconfigurations in SPF, DKIM, or DMARC—especially missing or incorrect 'include' entries—can prevent valid emails from being delivered even if the address is correct.

How can I test if my relay configuration is working?

Use a real-time email verification service to send test deliveries and validate whether valid addresses receive messages under your current routing setup.

Does Email List Validation check SPF or DKIM records?

No. It doesn’t analyze DNS records directly. Instead, it tests deliverability by simulating actual email delivery through your configured routing path.

Why should I use Email List Validation for routing validation?

It provides real delivery feedback: if an address is marked 'valid,' it means the message can be delivered through your current configuration, assuming DNS and infrastructure are sound.

What happens if my SPF record doesn’t include SendGrid or Mailgun?

Emails sent via these platforms will fail SPF authentication and are likely to be rejected by receiving mail servers, even if the address is valid.

Can I verify routing issues without sending real emails?

No. Only real delivery testing—using an API or inbox placement tool—can confirm whether routing configurations are working end-to-end.

How often should I validate my email list for relay-based issues?

After any major change to your email infrastructure, before large campaigns, and quarterly as a preventive check.

Does Email List Validation work with all email services?

It works with any service that uses standard SMTP or webhooks, including SendGrid and Mailgun. It doesn’t depend on the specific platform but on the deliverability path.

Can I automate validation of routing behavior?

Yes. The real-time verification API can be integrated into CI/CD or pre-send workflows to validate routing consistency across campaigns.

What does a 'valid' verdict mean in Email List Validation?

It means the address is likely to receive mail under current routing and deliverability conditions, assuming the domain's authentication is properly set.

Why do I get different results when testing the same email locally versus in bulk?

Local testing may use different routing paths. Bulk verification simulates your real infrastructure and catches misconfigurations that local tests miss.