Test Mailgun Domain in HubSpot | Email List Validation

Why Testing Mailgun’s HubSpot Integration Matters

You’ve connected Mailgun to HubSpot. The integration shows as active. But your campaign emails aren’t hitting inboxes—or worse, they’re bouncing with hard failures. It’s not always the list. Sometimes, the domain isn’t properly listed in Mailgun, and that small gap breaks everything.

Even a single misconfigured domain can trigger bounces, trigger spam filters, and damage sender reputation—especially during high-volume sends. The fix isn’t in your campaign copy or list hygiene. It’s in verifying that Mailgun’s DNS records are correctly set up to handle outbound mail from HubSpot’s platform.

To test if a domain is properly listed in Mailgun for HubSpot, you need to validate the setup through real-world deliverability checks—not just trust a green status badge. This article walks you through how to confirm your domain’s status, avoid inbox placement failures, and maintain sender reputation integrity.

Key takeaways

Even a seemingly active HubSpot-Mailgun integration can fail if the domain isn’t properly listed in Mailgun’s DNS configuration.
Unverified or misconfigured domains cause high bounce rates, which harm sender reputation and hurt deliverability.
Testing your Mailgun setup with real email send tests—before sending to production lists—prevents inbox placement failures and reputation damage.

What Does 'Properly Listed' Mean for Mailgun in HubSpot?

For Mailgun to be properly listed in HubSpot, it must be explicitly authorized as your sending domain in HubSpot’s SMTP settings, with your domain’s DNS records correctly configured. This means your SPF, DKIM, and MX records must all be valid and include Mailgun as an approved sender, and the domain must be verified in Mailgun’s dashboard without being flagged by spam filters.

SPF, DKIM, and MX — The DNS Foundation

SPF (Sender Policy Framework) is your domain’s official list of approved email senders. If Mailgun isn’t listed here, HubSpot’s outbound emails will fail authentication. DKIM (DomainKeys Identified Mail) adds cryptographic signing to emails so receiving servers can verify they haven’t been tampered with. Without a valid DKIM record, your emails risk being marked as spam. MX records define the mail server responsible for receiving messages. If they’re misconfigured or pointing to an unauthorized service, deliverability breaks down at the inbox level.

Let’s be clear: having these records isn’t enough. They must each be correctly set up and published in your DNS zone. For example, if your SPF record says include:mailgun.org but the include is missing or outdated, authentication fails. A misconfigured SPF can even lead to your domain being blacklisted. You can validate your DNS setup using tools like MxToolbox or RFC 7208 (the SPF specification).

Verification and Reputation

Even with perfect DNS records, Mailgun won’t accept your domain unless it’s verified in the Mailgun dashboard. You need to log in, add the domain, and confirm ownership — usually by adding a TXT record. Skipping this step means Mailgun won’t process your emails, regardless of SPF or DKIM. Once verified, that domain must also maintain a clean reputation. If it’s been involved in spamming, even briefly, spam filters could block your traffic.

It’s worth checking your domain’s reputation before integration. If your domain has ever triggered spam triggers — for example, high bounce rates or too many complaints — your sender reputation may already be harmed. The best way to avoid this? Clean your email list before sending. Use a tool like bulk email list cleaning to catch invalid, disposable, or risky addresses before they hurt deliverability.

How to Test if Your Domain Is Properly Listed in Mailgun for HubSpot

You can verify your domain’s Mailgun integration with HubSpot by checking the SMTP settings in HubSpot, confirming your domain’s SPF and DKIM records are properly published, and testing delivery with a real email. Use DNS tools to validate record resolution and real-time verification to ensure your domain’s MX is reachable.

Log into HubSpot and check your SMTP configuration. Go to Settings > Integrations > Email & Marketing > SMTP. Locate the Mailgun entry and confirm the domain listed matches the one you’re using. An incorrect domain will fail email delivery even if all DNS records are set.
Verify your domain's SPF record includes include:_spf.mailgun.org. This is critical—without it, receiving servers may reject your emails as unauthorized. Use a DNS lookup tool like MXToolbox to double-check the full SPF record resolves as expected.
Confirm DKIM keys are published and match Mailgun’s records. Mailgun provides a DKIM selector and public key. You must publish a TXT record with the selector (e.g., mailgun._domainkey) and the full key value. Use your DNS provider’s interface or a tool like RFC 6376 to validate the syntax.
Test email delivery through the HubSpot SMTP setup. Send a test email from HubSpot to a personal inbox. Check spam, junk, and promotions folders. If it doesn’t arrive, check your sender reputation and whether your IP is blacklisted on services like Spamhaus.
Use a real-time verification API to check MX record access. Not all domains with valid SPF and DKIM can receive mail. A test using a real-time verification API can confirm whether the domain’s MX record resolves correctly and whether the mail server is accepting connections. You can use real-time verification to test this at scale.

When in doubt, verify the full chain

Don’t assume DNS records are correct just because you entered them. Tools like MXToolbox and DNSChecker.org allow you to probe records from multiple locations globally. DNS propagation can take hours—verify after 1–2 hours if you’ve made changes.

Fixing issues proactively

If records are missing or malformed, you’ll see delays, bounces, or delivery failures. Fix SPF by updating the record to include include:_spf.mailgun.org. For DKIM, ensure the TXT record is published under the correct selector and matches what Mailgun expects. Use bulk cleanup to catch invalid domains before they impact deliverability.

How to Validate Mailgun DNS Records Using Free Tools

You can test if your domain is properly listed in Mailgun for HubSpot integration by verifying your DNS records with free tools: check SPF includes mailgun.org and stays under 255 characters, confirm DKIM has a matching TXT record with the correct selector (e.g., mailgun), and ensure your MX record points to mx.mailgun.org with priority 10. Use MxToolbox or command-line tools like dig and nslookup for real-time validation.

Check SPF Record Correctness

Go to MxToolbox and enter your domain to check your SPF record.
Ensure the record includes include:mailgun.org—this allows Mailgun to send on your behalf.
Verify the total length doesn't exceed 255 characters; SPF records over this limit are ignored by receivers.
If you have multiple include statements, consider merging or using DNS CNAME flattening to avoid breaches.

Confirm DKIM and MX Records

Find your DKIM selector (e.g., mailgun) from Mailgun’s domain settings.
Use MxToolbox or dig TXT mailgun._domainkey.yourdomain.com to check if a matching TXT record exists.
Validate that the key value matches exactly what Mailgun provides—any mismatch breaks authentication.
Run dig MX yourdomain.com or use MxToolbox to confirm the MX record lists mx.mailgun.org with priority 10.

These checks aren’t just formality—they’re how email providers verify authenticity. An incorrect SPF or missing DKIM leads to emails being flagged as spam or rejected entirely, especially for services like HubSpot that rely on consistent deliverability.

For teams managing multiple domains or automating verification at scale, real-time email validation tools help catch issues before they cause delivery failures. You can test domain-level issues with a bulk list cleaner before sending to HubSpot. Learn more about cleaning and validating large lists with our bulk verification tool, designed to catch invalid, catch-all, or risky domains early.

What Happens If the Domain Isn’t Properly Listed?

If your domain isn’t properly listed in Mailgun for HubSpot integration, outbound emails from HubSpot will likely be rejected with a hard bounce—often with a code like 550 5.7.1 Sender not allowed. Even if the message gets through, missing or incorrect DNS records can cause delays, spam filtering, or outright rejection. Over time, this damages your sender reputation, making future deliveries riskier—even for valid addresses.

Hard Bounces and Rejection at Scale

When Mailgun doesn’t recognize your domain as authorized, it blocks the sending IP from delivering mail. HubSpot then reports hard bounces. These aren’t just inconveniences—they count against your sender reputation with ISPs. A single bounce from a non-existent or unauthorized domain can trigger warning flags with providers like Gmail or Outlook.

Let’s say you send 1,000 emails and 300 get rejected because the domain isn’t set up correctly. That’s 30% of your campaign failing to reach inboxes. Even if your content is strong, your domain reputation takes a hit. You’re not just losing that one send—you’re increasing the odds that future emails will land in spam folders or be throttled.

Spam, Delay, and Reputation Risk

Even if Mailgun accepts the email, unconfigured DNS records like SPF, DKIM, or DMARC can cause delivery issues. Without them, emails are more likely to be flagged as suspicious. Some providers use reputation scoring systems—like those outlined in the SPF RFC—that factor in alignment and authentication history.

Missing or inconsistent records mean your messages get tagged as "risky." Gmail and other major services are increasingly aggressive about filtering low-reputation senders. Once your domain is seen as inconsistent or untrusted, even verified addresses can be routed to spam or delayed for inspection.

It’s not just about one campaign. A single misconfigured integration can erode trust over time. Tools like bulk email list cleaning help you avoid this by catching invalid or high-risk addresses before they hurt your metrics.

The root issue isn’t the email content—it’s the infrastructure beneath it. If your domain isn’t properly listed in Mailgun, the envelope fails before the message ever lands in an inbox.

Prevention Is the Real Fix

You can’t rely on HubSpot or Mailgun alone to catch DNS misconfigurations. Even small errors—like a typo in your SPF record—can cause rejection. The best way to prevent problems is to validate your entire email architecture, including authentication and domain alignment.

Testing your domain setup isn’t a one-time task. It should be part of every major integration and bulk send. Use tools that validate real-world deliverability, not just syntax. For instance, inbox placement testing checks where your messages land across major providers—giving you a real-world view of how your setup performs in practice.

How to Fix Common Mailgun-HubSpot DNS Issues

If your HubSpot emails to Mailgun are failing, the issue is likely in your DNS configuration. Check SPF, DKIM, and MX records. Ensure SPF includes both mailgun.org and your sender domain. Verify DKIM alignment by regenerating the key in Mailgun and updating the TXT record. Confirm MX records aren’t overridden by another service. Test the full delivery path in real time using an API that validates the entire email journey.

Check and Fix SPF Alignment

Go to your domain’s DNS settings and find the SPF record.
It must include both include:mailgun.org and your current sending domain (e.g., include:yourcompany.com).
If you’re using a third-party service like SendGrid or AWS SES, remove any conflicting records — SPF allows only one per domain.
Use RFC 7208 as a reference for proper SPF syntax.

Fix DKIM Signature Failures

Log into your Mailgun account and go to the Domains section.
Find your domain and click “Edit DNS” to regenerate the DKIM key.
Copy the new TXT record value and paste it into your DNS provider’s interface.
Wait up to 48 hours for propagation, then validate using a tool like MXToolbox.
DKIM fails if the record isn’t published or isn’t aligned with the sending domain.

Resolve MX Record Conflicts

Check your MX records via MXToolbox or your DNS provider’s portal.
Mailgun requires your domain’s MX to point to mx01.mailgun.org and mx02.mailgun.org.
If another service (like Gmail or Microsoft 365) has higher priority (lower number), it will override Mailgun.
Set Mailgun’s MX priority to 10 or lower; lower numbers mean higher priority.
Ensure no duplicate or conflicting MX records exist.

Validate the Full Email Path

Use a real-time verification API to test delivery from HubSpot through Mailgun to a real inbox.
Such testing covers DNS settings, server configuration, spam filtering, and inbox placement.
Tools like Email List Validation’s API simulate the full email journey and flag issues before they affect deliverability.
Testing in real time helps catch setup errors before sending to real users.

Use Email List Validation to Test Domains and Addresses

You can test whether a domain is properly listed in Mailgun for HubSpot integration by verifying its MX records, catch-all status, and SMTP-level deliverability using Email List Validation’s real-time API. This checks if the domain accepts mail, which is essential for consistent outbound delivery through Mailgun, especially during integration setup.

Check MX Records and SMTP Deliverability

Let’s be clear: a domain listed in Mailgun isn’t enough. It still must accept mail at the infrastructure level. Email List Validation sends a test email via SMTP to verify whether the domain’s mail server is active and willing to receive messages. It checks MX records for correctness and confirms that inbound mail is not silently dropped.

Many integration failures stem from misconfigured or non-receiving domains. By using SMTP validation, you catch these issues before they disrupt your HubSpot workflows. This avoids the “it worked once, then failed” problem where a domain appears valid but doesn’t deliver consistently.

Identify Catch-All and Risky Domains

Not all domains that accept mail are equally reliable. Email List Validation labels domains as valid, invalid, catch-all, or risky. A catch-all domain accepts all incoming mail, which means it won’t reject bad addresses—but it also opens you to spam traps and invalid data.

If your HubSpot integration sends to a catch-all domain, your sender reputation may degrade over time, even if the bounce rate seems low. That’s why catching catch-alls early is a must. Similarly, domains marked as risky may have greylisting, throttling, or high spam filtering rates—commonly seen in older or mismanaged domains.

For context, the [RFC 5321](https://datatracker.ietf.org/doc/html/rfc5321) (SMTP standard) defines how mail servers should handle incoming messages. Tools like Email List Validation follow these protocols to ensure accurate results. You’re not guessing—your integration health is based on real-time, protocol-compliant checks.

Use the real-time verification API to batch-check domains or test individual ones in seconds. It’s especially useful when onboarding new contacts or preparing a list for Mailgun-HubSpot sync. You’ll catch configuration issues, invalid domains, and risky senders before they impact deliverability.

Why Real-Time Verifications Beat Manual DNS Checks

You can verify DNS records all day, but that doesn’t mean Mailgun will accept your messages—even if SPF, MX, and DKIM are technically correct. Issues like greylisting, temporary delivery blocks, or sender reputation issues can cause bounces despite pristine DNS. A real-time verification API simulates actual message delivery, catching these live problems before you send. It’s not about static records—it’s about real behavior.

Making sure your domain’s MX record points to Mailgun is a necessary first step. But SPF and MX checks alone don’t confirm whether Mailgun will actually ingest your email. Some domains pass DNS checks but are still on greylists, blocked by anti-spam systems, or have throttling rules in place. These issues only surface during an actual SMTP handshake.

For example, if Mailgun’s server temporarily rejects a connection due to high load or rate limiting, a DNS-only tool won’t detect it. That same message might fail later when sending to a real user. This is why you need more than just a record checker.

Simulating Delivery Catches Hidden Failures

A real-time API like the one offered by Email List Validation runs actual SMTP connections to the recipient’s mail server. It checks whether Mailgun accepts the envelope, processes the message, and allows delivery—to an actual inbox or not. It does this across multiple check points in the transaction chain, catching issues that static tools miss.

For instance, while a service might show a valid MX record, the domain could be flagged by a sender reputation system like Spamhaus. That domain might still be blocked in practice, even if the DNS says otherwise. Real-time validation exposes these conditions early.

This approach has been shown to detect deliverability risks with higher accuracy than DNS-only methods. A study on email deliverability patterns by Return Path (now Validity) found that 14% of delivered emails were blocked due to transient issues that static checks can’t spot. Let’s be honest: relying on DNS alone is like driving with the dashboard lights off—you might not see the warning until it’s too late.

The Email List Validation API performs these checks at 98.9% accuracy, meaning it reliably separates the domains that will work from those that won’t, even when DNS is clean. It doesn’t guess. It tests.

Run a real-time verification on your Mailgun-connected domains to test inbox placement, sender reputation, and connection acceptance—all before you send.

Test Your HubSpot to Mailgun Flow with Inbox Placement

Send a test email through your HubSpot-to-Mailgun setup to 10 major mailbox providers—Gmail, Outlook, Yahoo, Apple Mail, and others—to check inbox placement, spam score, and delivery reliability. This simulates real-world delivery and exposes issues before you send to your full list. If any email fails or lands in spam, you’ll know exactly where the breakdown occurs.

Run a Deliverability Test

Use inbox placement testing to send a sample message from your Mailgun setup through HubSpot to popular inboxes like Gmail, Outlook, and ProtonMail. This tests how your domain, IP, and content perform in actual mail delivery environments. According to industry standards, mailbox providers use complex filtering systems that go beyond SMTP checks—so only real inbox delivery tests reveal true results.
Review the results for inbox placement rate (e.g., how many landed in the inbox vs. spam), spam score (a numeric risk rating), and delivery errors (like timeouts or rejection codes). A score above 8/10 on spam risk is typically considered high-risk.
Identify root causes if delivery fails or spam scores are high. Use a verification API to isolate whether the problem is a misconfigured domain, a poor sender reputation, bad content, or an invalid recipient. For example, if emails to certain domains fail consistently, the issue may lie with Mailgun's DNS records or your SPF/DKIM setup.

Fix the issue before it hits your list

Let’s say your test shows 40% inbox placement and a high spam score. You could have a sender reputation issue, misaligned authentication, or content triggers. Fixing the problem early saves time, reduces bounces, and protects your brand reputation.

Detecting delivery issues before sending at scale is standard practice in email operations. Testing across real inboxes—one of the only ways to validate deliverability—is supported by deliverability best practices from IETF and industry leaders like Mail-Tester and Return Path.

With real-time email verification, you can pre-screen recipient addresses to rule out invalid or risky domains. This reduces failed deliveries and keeps your sender reputation strong. Test your email list with a tool that validates addresses against 40+ delivery rules in real time—before they ever hit Mailgun.

Common Mistakes That Break Mailgun-HubSpot Integration

You don’t just add a domain to Mailgun and expect HubSpot to send through it. If you haven’t verified the domain in HubSpot’s SMTP settings, set up DNS records properly (SPF, DKIM, DMARC), and tested real sends, your emails will still fail. Even a single mismatched or outdated DNS record can break delivery. Let’s walk through the most common missteps that silently sabotage integration.

SMTP Setup & DNS Configuration Gotchas

Adding a domain to Mailgun without configuring it in HubSpot’s SMTP settings will result in delivery failures, even if DNS is correct. You must configure the same domain in both platforms to establish trust.
Legacy SPF records that don’t include include:mailgun.org block Mailgun’s authentication. If your domain uses multiple email services, the SPF record must explicitly allow Mailgun or it will be rejected by receivers. Refer to RFC 7208 for valid SPF mechanisms.
Overwriting DNS records during migrations without testing the full chain is a frequent cause of breakage. A single missing or incorrect TXT or MX record can lead to bounces or spam placement.

Assumptions That Lead to Failure

Assuming that setting up the integration means emails will send successfully is a major error. Configuration ≠ delivery. Always test with real messages sent from HubSpot to known email addresses.
Ignoring deliverability signals like bounce rates, spam complaints, or inbox placement. A clean setup doesn’t guarantee inbox delivery—your sender reputation matters too. Use tools that simulate real-world delivery to verify placement.
Not verifying your email list before sending. Invalid, fake, or outdated addresses increase bounce and spam rates, harming your sender reputation and increasing the chances of being blocked.

These aren’t just technical hiccups—they’re systemic risks that erode sender trust. Even if Mailgun accepts the message, a bad list or misconfigured domain can still result in delivery failure or spam filtering.

If you’re sending at scale, start by cleaning your list: verify every address before sending. A 98.9% accurate email verification tool can help spot invalid, disposable, or risky addresses before they hurt your deliverability.

Ensure Reliable Email Delivery with Proactive Testing

Domain configuration isn’t a one-time task. It’s a live system that changes with DNS updates, email provider policies, and sender reputation shifts. Treat it as such.

Don’t rely on DNS checks alone. Combine them with real-time verification and inbox placement tests to confirm your domain is not just listed, but trusted by major providers.

Email List Validation helps you audit list health and domain validity continuously. No credits expire. Start with 100 free verifications to test your Mailgun-HubSpot setup, validate your domain, and ensure your messages reach inboxes—without surprise bounces or blocks.

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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

Can I test a Mailgun domain in HubSpot without sending actual emails?

Yes. Use tools like MxToolbox or Email List Validation’s real-time API to verify DNS records and domain delivery without sending messages.

What SPF record should I use for Mailgun with HubSpot?

Your domain’s SPF record must include both your current sender domain and include:_spf.mailgun.org.

How does Email List Validation test if a domain is properly listed?

It performs real-time SMTP checks to confirm the domain accepts mail, validates MX and DKIM records, and tests inbox placement through major providers.

Why might my Mailgun domain pass DNS checks but still fail in HubSpot?

Because DNS records are static; real delivery depends on dynamic factors like greylisting, blocklists, or temporary server issues.

Does Mailgun require domain verification before use?

Yes. You must verify ownership of the domain in Mailgun’s dashboard to enable sending.

Can I use multiple domains with Mailgun in HubSpot?

Yes, but each domain must be individually verified in Mailgun and configured in HubSpot’s SMTP settings.

What’s the difference between SPF, DKIM, and MX records?

SPF authorizes sending sources, DKIM adds message signatures for authenticity, and MX routes inbound mail to the correct server.

How accurate is Email List Validation’s domain check?

It achieves 98.9% accuracy by combining real-time verification and DNS analysis at scale.

Can I verify a domain without using an email address?

Yes—Email List Validation’s domain check uses DNS and SMTP-level probes without requiring a target email.

Do I need to retest my domain after changing DNS settings?

Yes—DNS changes take time to propagate. Retest after 24 hours to confirm the new configuration is live.

Can disposable domains mess up HubSpot integration?

Only if they’re used as sending domains. Mailgun and HubSpot prevent sending from disposable domains by default.

What if my domain is listed on a blocklist?

Use Email List Validation’s inbox-placement test to check spam scores and deliverability risk before sending.