How to Verify Sender Domain in HubSpot When Mailgun Returns 550 5.1.3

Why does Mailgun return 550 5.1.3 when sending through HubSpot?

You send a campaign through HubSpot using Mailgun as your SMTP relay. The delivery fails with a 550 5.1.3 error. Not a bounced address. Not a typo. The domain itself is being rejected.

This isn’t about the email address being wrong. It’s about the domain’s reputation and configuration. The recipient’s mail server sees your sender domain as invalid or unverifiable. It’s like showing up at a gated building with a fake ID—your message gets blocked at the door.

Understanding and fixing this error starts with sender domain verification in HubSpot—not just the email, but the domain behind it. That’s exactly what we’ll cover here: how to verify sender domain in HubSpot when Mailgun returns 550 5.1.3.

Key takeaways

• 550 5.1.3 means the recipient server rejected the sender domain as invalid or non-routable.
• HubSpot’s sender domain must be verified and properly authenticated with SPF, DKIM, and DMARC records.
• Failure often stems from missing or misconfigured authentication records, not the email address itself.

How to verify sender domain in HubSpot when Mailgun returns 550 5.1.3

When HubSpot shows a 550 5.1.3 error with Mailgun, the sender domain isn’t properly verified in HubSpot’s SMTP settings. You must confirm the domain listed in HubSpot matches the one used in Mailgun, that DNS records (SPF, DKIM) are correctly published, and that HubSpot can connect via SMTP. A mismatch here often triggers the error.

Step-by-step verification process

1. Log in to HubSpot and go to Settings > Integrations > Email. This is where HubSpot manages all outbound email configurations, including SMTP relaying. Make sure you’re in the correct account and portal.
2. Navigate to 'Send email via SMTP' and verify the sender domain. The domain listed must exactly match the one used in Mailgun’s sending settings. A typo or mismatch — like using example.com in HubSpot and mail.example.com in Mailgun — will fail validation.
3. Confirm the domain is set up for outbound email in HubSpot’s SMTP config. The domain must be added to HubSpot’s allowed domains list under SMTP settings. If it’s not listed, you’ll get a 550 5.1.3 error because the mail server rejects the sender identity.
4. Run HubSpot’s built-in SMTP test. Use the “Test connection” button in the SMTP settings. If the test fails, it means either the configuration is wrong or DNS records aren’t properly published. Test results will show whether the domain is recognized and accepted.
5. Check DNS records for SPF, DKIM, and MX alignment. Ensure your Mailgun domain has valid SPF records allowing HubSpot’s IPs to send on its behalf. DKIM should be signed and published. Mismatched or missing records can cause rejections, even if the domain name is correct.

When the test fails: common causes

A failed SMTP test usually points to one of two issues: misconfiguration or missing DNS records. SPF and DKIM must be published and correctly formatted — a single typo in an SPF record can break authentication. Use tools like MxToolbox or dmarcanalyzer.com to verify DNS entries. You can also check your Mailgun dashboard for active sending domains and ensure they’re in sync with HubSpot.

Let’s be clear: the 550 5.1.3 error isn’t a HubSpot bug — it’s a sender identity rejection. If everything matches and tests still fail, your Mailgun account might need a manual domain verification or IP warm-up. Always double-check the exact domain name used in both systems. For ongoing list health and sender reputation tracking, consider using a bulk verification tool like bulk email list cleaning to catch invalid or risky addresses before sending.

Common causes of 550 5.1.3 errors in HubSpot + Mailgun setups

When HubSpot sends via Mailgun and gets a 550 5.1.3 error, it usually means the recipient server rejected the message because your sending domain’s authentication isn’t properly set up. This error points to missing or incorrect SPF, DKIM, or DMARC records, an unverified domain in HubSpot, or Mailgun blocking your domain due to reputation issues. Let’s go through the most common culprits and how to fix them.

Authentication misconfigurations

• Check that your sender domain in HubSpot has a valid SPF record allowing Mailgun’s IP ranges to send emails on its behalf. If it doesn’t, recipients will reject messages. You can verify this using MXToolbox or similar tools that test SPF records.
• Ensure DKIM signing is enabled and properly configured in your Mailgun domain settings. Without it, emails fail to pass authentication checks, especially with major providers like Gmail or Outlook.
• Review your DMARC policy. If it’s set to reject and your domain doesn’t have a DMARC record or the policies don’t align with SPF/DKIM results, your messages get blocked. Use RFC 7483 as a reference for DMARC implementation standards.

Domain verification and reputation issues

• Confirm your domain is verified in HubSpot’s Sending Domain settings. If you recently changed domains or added a new one, it must be re-verified manually in HubSpot’s email settings.
• Check if your Mailgun domain has been flagged for spam or has a poor sender reputation. Mailgun may block domains with recent abuse reports, high bounce rates, or inconsistent sending behavior. You can review your sender reputation via Mailgun’s built-in metrics dashboard.
• Verify that Mailgun has not restricted your domain due to volume or compliance issues. High-volume sending without proper authentication is a red flag.

If you're unsure whether your domain can send reliably, test it with a real inbox placement tool. You can get a realistic view of whether your messages are landing in inboxes or spam folders. Test inbox delivery before large campaigns to avoid delivery failure at scale.

What happens if you ignore a 550 5.1.3 error?

You’ll block all email delivery from that domain in HubSpot until the underlying issue is resolved. A 550 5.1.3 error means the recipient’s mail server explicitly rejected your message because the sending domain is not authorized to send from the sender address. Ignoring it means every outbound email from that domain fails, damaging your sender reputation and potentially leading to broader delivery issues across other providers.

Delivery fails completely

If the domain isn’t properly configured in HubSpot — particularly if SPF, DKIM, or DMARC aren’t set up correctly — your emails get rejected at the gateway. The 550 5.1.3 error isn’t a temporary glitch; it’s a hard rejection. Even if the email looks valid, the server won’t accept it, and you’ll see a permanent bounce.

Let’s be clear: you can’t send one successful message and then ignore the error. Once a domain is marked as unauthorized by a recipient’s server, every subsequent email from that domain gets treated the same way. This isn’t just a Mailgun problem — it’s a domain-level issue affecting all providers. If the domain is misconfigured, every gateway will reject it.

Reputation and deliverability suffer

Your sender reputation degrades quickly when you persist with sending to invalid or rejected domains. According to industry standards from RFC 5321, persistent delivery failures indicate poor list hygiene or technical misconfiguration. Over time, ISPs and mailbox providers like Gmail and Outlook will lower your sender score, reducing inbox placement across the board.

High bounce rates — even if they’re only from one provider — are red flags. They signal to email networks that your list is low-quality, which can trigger automated filtering. This affects delivery even for valid emails sent from other domains. One misconfigured sender domain in HubSpot can pull down your entire sending reputation.

Customers who don’t receive critical updates, order confirmations, or password resets lose trust. In high-engagement industries like e-commerce, recurring missed delivery leads to churn. You aren’t just failing to reach people — you’re actively eroding confidence in your brand’s reliability.

Before you assume the issue is with Mailgun or HubSpot, verify that the domain itself is correctly set up. Use real-time domain validation tools to catch these problems early. You can check domain configuration and sender reliability with an email verification API or clean your entire list with bulk verification to prevent sender domain issues from spreading.

Use Email List Validation to catch sender domain issues before they break delivery

When Mailgun returns a 550 5.1.3 error, it often means your sender domain is misconfigured or blocked. Email List Validation catches these issues upfront by performing real-time SMTP checks against your domain, verifying MX, SPF, DKIM, and DMARC records, and scanning for blacklists — so you fix problems before they tank your deliverability. You’re not guessing; you’re testing with actual email infrastructure.

Real-time SMTP checks go beyond DNS

Many tools stop at checking DNS records, but Email List Validation connects directly to the mail server using real SMTP sessions. This means it doesn’t just confirm a domain exists — it checks whether mail from that domain will actually be accepted. This process catches issues like missing or broken SPF records, misconfigured DKIM signatures, or failed DMARC policies that silently block sends.

It finds traps your tools miss

Some domains appear valid on paper — they have working MX records and proper SPF — but are blocked by major providers like Gmail, Yahoo, or Outlook. These domains may be on the spam blacklist, have temporary greylisting, or be flagged due to poor sender reputation. Email List Validation scans against known blocklists, including those maintained by Spamhaus, and identifies domains at risk of being rejected. This is not just about format — it's about whether the domain is trusted in practice. Spamhaus is one of the most widely used reputation databases for email filtering, and being listed there can stop all delivery.

Another subtle issue: catch-all domains. These accept any email address at the domain, which can cause misrouting, increase spam marking, and hurt your sender reputation. Email List Validation flags these configurations so you know which domains to avoid or exclude before sending.

Let’s say you're using HubSpot with Mailgun for transactional email. You’ve verified the domain, but still see 550 5.1.3. The problem might not be HubSpot — it could be that your domain’s SPF allows Mailgun but not your actual sending IP, or that your domain is blocked due to previous abuse. Email List Validation lets you verify the full stack in one go: sender domain, authentication setup, and real-time rejection risk.

Unlike tools that only validate email addresses, Email List Validation checks the entire sending ecosystem. It’s designed for teams that need to deliver consistently at scale — whether you're sending in bulk, automating campaigns, or managing a large CRM list. With 98.9% accuracy, it helps you clean your list before send, reducing bounces and protecting your reputation.

Try a real-time verification to catch domain-level issues before they break your pipeline. Verify your sender domain live — no credit card required.

How does Email List Validation compare to HubSpot’s built-in domain verification?

You can't rely on HubSpot’s domain verification to confirm that Mailgun can actually send emails through your domain. It only checks basic DNS records like MX and SPF — not whether the domain accepts mail from your sender IP or if messages are being blocked silently. Email List Validation goes deeper by simulating real SMTP transactions, detecting bounces before you send, and analyzing sender reputation. This reveals domains that claim to accept mail but never deliver, something HubSpot’s check misses entirely.

What HubSpot’s domain verification actually checks

• HubSpot verifies if your domain has valid DNS records — specifically MX, SPF, and sometimes DKIM — but does not test if the receiving server accepts incoming mail from your configured IP.
• It won’t detect if a domain uses greylisting, has a sender policy that blocks third-party services, or if delivery is blocked on reputation grounds, even if the server responds to DNS queries.
• You might see "verified" in HubSpot, but still get 550 5.1.3 errors from Mailgun due to undisclosed sender restrictions or policy mismatches.

Why Email List Validation catches what HubSpot misses

• Unlike HubSpot, Email List Validation performs real-time SMTP simulations — it connects to the mail server, initiates the transaction, and observes the response, including silent rejections.
• It identifies domains that respond to DNS checks but reject mail from your specific sender IP, including those using catch-all configurations that pretend to accept all mail but never deliver.
• It flags domains with poor sender reputation, those on blocklists, or those that enforce strict filtering policies — all of which contribute to 550 5.1.3 errors even with correct DNS.
• It checks for role-based addresses (like admin@ or info@) that are often auto-rejected by modern email filters, reducing your deliverability risk before you send.
• Each verification includes a verdict: valid, invalid, catch-all, risky, or disposable — with clear reasons, not just a green checkmark.

For example, a domain may have a working MX record but reject all mail from non-authorized IPs — a common cause of the 550 5.1.3 error. This is invisible to DNS-only checks. By simulating actual delivery attempts, Email List Validation exposes these gaps.

SMTP transaction testing is an industry-standard way to verify actual delivery feasibility — outlined in RFC 5321, the core protocol for email transmission. You can read more in the official SMTP specification.

Use bulk email list cleaning to test your full subscriber list before sending, ensuring you only send to domains that accept Mailgun mail — and avoid 550 5.1.3 errors before they happen.

What verdicts does Email List Validation return for domain verification?

You’ll get one of four verdicts when verifying a sender domain: Valid, Invalid, Catch-all, or Risky. A Valid domain accepts mail and passes authentication checks. Invalid means the domain outright rejects messages. Catch-all domains accept all emails, increasing spam risk. Risky indicates weak authentication, poor sender reputation, or recent delivery issues. These verdicts help you avoid bounces and protect your deliverability, especially when tools like Mailgun return a 550 5.1.3 error.

Understanding Each Verdict

Let’s break down what each result means in practice and how it affects your email campaigns.

These verdicts aren’t just labels — they’re based on real-time SMTP checks, DNS record analysis, and historical sender behavior. Unlike some tools that offer only a binary "valid/invalid" result, Email List Validation provides a nuanced assessment that helps you make data-driven decisions.

For example, a Mailgun 550 5.1.3 error often appears when a domain’s MX records are misconfigured or the sender has been blocked. A “Risky” verdict from Email List Validation can surface that issue before you send — helping you catch it early and avoid hard bounces.

If you’re verifying large lists or need to automate checks, our real-time API integrates with systems like HubSpot and SendGrid. Or, if you’re cleaning a long-term list, try our bulk verification tool, which includes all four verdicts in a clean, actionable report.

How to integrate Email List Validation with HubSpot to prevent 550 5.1.3 errors

If Mailgun returns a 550 5.1.3 error when sending through HubSpot, it’s often because your sender domain isn’t properly authenticated or your list contains bad addresses. You can stop these errors by verifying all emails before import, using the Email List Validation API to clean your list, syncing only valid addresses to HubSpot, and testing inbox placement after import to ensure deliverability.

1. Verify your entire list using the Email List Validation API before uploading to HubSpot. This checks syntax, domain validity, and mailbox existence. It catches errors early—like mistyped addresses or non-existent domains—before they trigger SMTP rejections. This step prevents 550 5.1.3 errors caused by invalid or non-reachable addresses.
2. Run an inbox placement test after importing into HubSpot. Use the inbox placement tool to send test emails to real Gmail, Outlook, and Yahoo inboxes. This shows you how your messages appear in real user folders, not just test environments. According to return path data, only 79% of transactional emails reach the inbox across all providers, so testing is not optional.
3. Sync verified data directly into HubSpot via the integrations feature. Once your list is clean, use the integration to push only valid contacts into your HubSpot database. This avoids importing invalid emails that damage sender reputation. Real-time sync ensures your CRM always has accurate data.
4. Filter out any domain flagged as 'Risky' or 'Catch-all' before sending. A catch-all domain accepts all emails, making it a high-volume spam risk. Risky domains may be associated with poor deliverability or high bounce rates. Removing them reduces bounces and protects your sender reputation.

Why this prevents 550 5.1.3 errors

SMTP 550 5.1.3 errors typically mean the recipient’s server rejects the message because the email address doesn’t exist, the domain is invalid, or the sender is not authorized. While Mailgun may return this error due to SPF, DKIM, or DMARC misconfigurations, it’s often compounded by bad data. Cleaning your list before sending reduces these failures and improves your sender reputation. You can validate domain configuration with tools like MXToolbox or check DNS records via the SMTP RFC.

Next steps: maintain deliverability

After integration, continue monitoring deliverability. Use real-time verification for new leads and perform quarterly inbox placement tests. Keep your list updated—not just to avoid 550 5.1.3 errors, but to ensure messages actually reach customers.

What if your domain is rejected but appears valid in DNS?

Just because your domain passes DNS checks doesn't mean Mailgun will accept your emails. If you're getting a 550 5.1.3 error despite valid SPF, DKIM, and MX records, the issue is likely reputational: your sending domain may be blacklisted, flagged for spam traps, or has a poor engagement history. Even technically correct DNS won't override a sender reputation hit. Let’s break down why and what you can do.

Reputation matters more than DNS alone

Domain-level validation only confirms technical readiness, but not deliverability. A domain can have perfect DNS records and still be blocked if it’s associated with spam, high bounce rates, or low engagement. The 550 5.1.3 error typically signals that the recipient server knows your domain — but doesn’t trust it. This is common with newly registered domains or those previously used for bulk sends.

Blacklists like Spamhaus or AbuseIPDB track not just IP addresses but linked domains. Even if your IP is clean, a domain with a history of abuse can still be rejected. Some providers, including Mailgun, use real-time reputation scoring that evaluates prior sending behavior and engagement — not just DNS syntax.

Catch issues before they hit your inbox

Running a basic DNS check misses the real transactional risks. You need a tool that validates at the SMTP level — simulating a real email transaction to see if a domain will accept mail under live conditions.

Email List Validation performs transaction-level checks across major email providers. It doesn’t just scan DNS; it attempts the full SMTP handshake, identifying not just invalid domains, but also those blocked due to reputation, catch-all configurations, or greylisting. This avoids sending to domains that appear valid on paper but are unreachable in practice.

For teams using HubSpot with Mailgun, this means catching send failures early—before they clog your queue or hurt deliverability. You can run a full bulk verification of your list to identify domains with hidden blacklisting, poor sender reputation, or temporary delivery issues. This is especially useful when scaling outbound campaigns.

Rather than relying on DNS alone, test your domain and list with a service that checks real-world acceptance. Bulk email list cleaning helps you weed out risky domains before sending, reducing bounces and protecting your sender reputation.

For deeper visibility, inbox placement testing shows where your messages land in real inboxes—whether in the inbox, spam folder, or blocked entirely—based on how providers treat your domain today.

Technical validity is a baseline. Deliverability is about trust. And trust is earned through sender reputation and consistent sending behavior. DNS says “you are here.” SMTP says “we’ll let you in.” Make sure your domain passes both.

Why real-time email verification beats deferred fixes

You don’t fix sender reputation by cleaning bounces after the fact. Real-time verification catches invalid, dormant, or risky addresses before they’re sent—cutting bounce rates by up to 95% in typical campaigns and preventing damage to your sender reputation. Sending to hundreds of dead or disposable emails wastes bandwidth, spikes rejection rates, and harms deliverability over time. The fix isn’t post-send cleanup—it’s pre-send validation.

Fixing bounces is reactive, not preventive

When Mailgun returns a 550 5.1.3 error, it’s already too late. Your message was rejected, your sender IP took a hit, and the bounce will likely be logged by blacklist providers. Bounces like this signal a problem with your domain’s reputation or a non-existent address—but the damage is done. You’re already on the wrong side of sender reputation systems like those maintained by Return Path, which track sending behavior over time (Return Path). Fixing them after the fact is like patching a leak after your ship has sunk.

Preemptive validation stops issues before they start

Instead of waiting for bounces, validate your list in real time. Tools like Email List Validation use SMTP checks, MX validation, and role account detection to identify real problems—invalid formats, non-existent domains, catch-all addresses, or disposable email services—before you send. With 98.9% accuracy, you’re not just filtering out noise—you’re identifying persistent risks that would otherwise harm deliverability. The result? Campaigns that land in inboxes, not dumps.

Let’s say you’re sending to 10,000 contacts. Without verification, you could be wasting resources on hundreds of dead addresses or temporary ones that won’t engage. With real-time validation, you reduce that number to a handful—cutting unnecessary sends, avoiding reputation penalties, and improving open rates. It’s not a magic fix, but it’s the only technically sound way to protect deliverability at scale.

You don’t need to solve every 550 error manually — automate and prevent

When Mailgun returns a 550 5.1.3 error, it’s a signal that your sender domain or recipient email is invalid. Instead of diagnosing each case by hand, automate verification upfront.

Email List Validation’s API checks every email in a new list import before it hits HubSpot. This stops invalid or risky domains from entering your campaigns, reducing bounces and protecting sender reputation.

Integrate verification into your workflow: flag high-risk domains, test inbox placement in real-world conditions, and maintain a clean list without relying on post-send audits. Prevention is faster, more reliable, and scales with your outreach.

Keep reading

• Email marketing compliance: GDPR, CAN-SPAM, consent and unsubscribes (complete guide)
• How to Ensure Domain Authentication in Mailgun for HubSpot Email Sync
• Fix 554 5.4.2 Content Filtering Bypass Failure with Domain Reputation Checks
• Enterprise Email Verification Platform to Avoid 550 5.7.1 Spam Rejection Errors
• 550 5.7.1 Authentication Failed After Switching Email Providers

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

Frequently asked questions

What does Mailgun 550 5.1.3 mean when sending through HubSpot?

It means the recipient’s server rejected the sender domain as invalid or unrecognized. This usually points to misconfiguration in DNS, SP, or DMARC records.

Can HubSpot verify a sender domain for SMTP use?

Yes, HubSpot verifies domain ownership through DNS MX and TXT records, but it does not validate whether the domain accepts mail from Mailgun in real time.

Is Email List Validation a better way to verify domains than HubSpot’s tool?

Yes. Email List Validation performs live SMTP verification and checks sender reputation, while HubSpot only validates basic DNS records.

How accurate is Email List Validation’s domain verification?

It achieves a 98.9% accuracy across bulk and real-time checks, using real-time transaction simulations that reflect actual delivery behavior.

Can Email List Validation fix a 550 5.1.3 error?

It cannot fix misconfiguration, but it identifies the root cause — such as invalid domains, catch-alls, or poor sender reputation — before you send.

Do I need to verify every domain in my list?

Yes. Even if one domain in your list is blacklisted or misconfigured, it can harm your overall sender reputation and trigger 550 errors.

What’s the difference between a catch-all domain and an invalid one?

A catch-all accepts all emails, which increases spam risk. An invalid domain rejects all mail, leading to immediate bounce errors.

How often should I verify my email list in HubSpot?

Perform verification before every major campaign or list import. Use automated checks for ongoing list hygiene.

Does Email List Validation integrate with HubSpot?

Yes, with direct sync support. Verified data can be pushed into HubSpot contacts and used to filter out weak senders.

Is there a free way to test Email List Validation?

Yes. Start with 100 free verifications. Credits never expire, so you can test at your own pace with no time pressure.