Why Deliverability Fails—Even with Mailgun

You sent 10,000 emails through Mailgun. Your deliverability dashboard says 98% landed in inboxes. But why are your open rates flat? Why are some customers reporting they never got your message?

Mailgun excels at routing and delivering mail—but it doesn’t tell you if the address on the other end is still alive, active, or even real. A bounce isn’t always a spam filter. It’s often a bad address that never should have been in your list.

Think of Mailgun like a postal service that delivers every letter you hand it—whether the recipient still lives there, has moved, or is a fake name. Without verifying each address first, you’re sending on faith, not data. A step-by-step guide to verifying email deliverability with Mailgun integration starts not with the send, but with the address.

Key takeaways

  • Mailgun handles delivery but doesn’t validate email addresses before sending.
  • High bounce rates often stem from invalid or dead addresses, not spam filters.
  • Verifying emails before sending prevents wasted sends and protects sender reputation.

The Real Step-by-Step Guide to Verifying Email Deliverability with Mailgun

Let’s cut through the noise. You’re using Mailgun to send high-volume emails. But how do you know your messages are landing in inboxes, not spam folders or voids?

Set Up the Integration

  1. Add your Mailgun domain to MailTester’s dashboard. Go to MailTester’s integrations page and link your Mailgun domain. This allows MailTester to assess your sending reputation, authentication setup (SPF, DKIM, DMARC), and historical deliverability trends. Real sender reputation is non-negotiable — even the best content fails if the infrastructure is weak.
  2. Upload your list or integrate via the real-time API. Use the bulk verification tool for static lists, or connect through the real-time verification API for on-the-fly validation in your workflow. The API supports 100 free verifications upfront — no expiration on purchased credits.
  3. Run an inbox-placement simulation. With MailTester’s inbox placement test, simulate delivery across provider-specific filters (Gmail’s spam algorithm, Outlook’s risk scoring). This isn’t a guess — it’s a data-backed simulation based on real inboxing behavior patterns observed by third-party monitoring services.
  4. Filter out invalid, catch-all, and risky addresses. MailTester returns exact verdicts: valid, invalid, catch-all, or risky. Catch-all domains (where any address is accepted) can skew engagement metrics and trigger spam filters. The 1.4% average bounce rate for unverified lists — common in email marketing — can be reduced to under 0.5% with proper filtering.
  5. Resend only validated addresses through Mailgun. Don’t send to the “risky” or “catch-all” group. Send only to confirmed valid addresses. This reduces sender reputation risk and improves open/click rates, which directly influence how future messages are treated by mailbox providers.
  6. Monitor and update your list quarterly. Email lists decay. Use MailTester’s repeatable verification cycles to keep your list clean. Re-verify every 90 days to prevent hard bounces, protect deliverability, and maintain alignment with RFC 5321’s standards for email validation.

For teams building custom workflows, the email finder complements verification by helping you source new, valid contacts from company domains.

Deliverability isn't set and forgotten. It’s an ongoing process tied directly to list hygiene and authentication integrity.

Mailgun’s infrastructure is solid. But without verified addresses and clean data, even the best setup fails. This process ensures you’re sending only to valid inboxes — not to systems that can’t receive, reject, or mark your messages as spam.

What Each Verification Verdict Means for Mailgun Sending

When you integrate Mailgun with MailTester, you’re not just checking emails—you’re making smart sending decisions. Each verdict has a real impact on deliverability, list health, and sender reputation. Let’s break down what each one means in practice, so you know exactly what to do with every address.

Understanding the Verdicts

Verifying your list is the first step to sending at scale without triggering spam filters or losing reputation. Here’s what each result means—from the ground up.

Verdict What It Means Mailgun Action
Valid The email address exists and is accepting messages. Server-level checks passed, and no red flags. Good to send. Proceed with your campaign or transactional flow. These are your core recipients.
Invalid Address syntax is broken, domain doesn't exist, or the mailbox is permanently non-deliverable. Remove immediately. Sending to invalid addresses increases bounce rates and harms your sender score. Bulk verification helps catch these fast.
Catch-all The domain accepts any email—even random strings. This often means spam traps or abuse. Mark as risky. Avoid sending high-volume or promotional content. Catch-alls can generate false positives and flag your IP if used at scale.
Risky Address is technically valid but likely to be flagged by filters due to past abuse, poor engagement, or low reputation. Send cautiously. Use in low-sensitivity campaigns only. Consider segmenting these into a separate list for monitoring.
Disposable The email is temporary—created for sign-ups and will expire within hours or days. Do not send transactional content. These are often used for one-time verification, not long-term engagement.

Not all emails are created equal. A “valid” email today might become a source of complaints tomorrow if the user never engaged. This is why ongoing list hygiene is critical. Tools like Mailgun track deliverability over time, but you can't depend on them to catch bad addresses upfront.

For deeper insight, run a real-time inbox placement test with MailTester’s inbox placement feature. It simulates how your message lands across Gmail, Outlook, and other major inboxes—giving you a real-world preview before you send.

According to RFC 5321, SMTP requires a valid recipient before accepting delivery. But that doesn’t mean the recipient will ever read it. That’s where verification comes in—filtering out the noise before it hits the wire.

Think of MailTester as your pre-flight check: it doesn’t fly your email for you, but it tells you if the plane’s ready to go. With accurate verdicts and real-time API access, you can automate clean list flows into Mailgun—no guesswork, no reputation damage.

How MailTester Works with Mailgun's SMTP and API

You’re using Mailgun to send emails. Good. But what if you’re sending to invalid, risky, or disposable addresses before they even hit your inbox? That’s where MailTester comes in — not as a replacement for Mailgun, but as a guardrail before your sends ever begin.

Pre-Send Checks with the Real-Time API

Let’s say you collect emails on a form. Instead of sending them straight to Mailgun, you run them through MailTester’s real-time API. It checks validity, spam risk, and domain health before they reach your Mailgun SMTP endpoint.

This happens in milliseconds. You don’t slow down your user journey. You just get a clean signal: valid, risky, catch-all, or invalid.

And if you're building an app, you can integrate MailTester’s API silently into your data pipeline. It logs results. Flags suspicious addresses. No manual review needed.

For example, SMTP RFC 5321 defines how email servers handle delivery, but it doesn’t prevent sending to non-existent addresses. That’s where proactive verification helps. Real-time checks catch the problem before it becomes a bounce.

Bulk Verification: Clean Your List Before Sending

Maybe you have a 5,000-person list with outdated or typo-ridden emails. Sending it all through Mailgun floods your inboxes with bounces and strains sender reputation. MailTester’s bulk verification runs through that list in minutes.

It checks each address against the same standards as real-time verification. You get a report showing what’s valid, what’s risky, and what’s dead. Then you remove the noise before sending.

Now, when you do send via Mailgun, you're sending to a tighter list. Fewer bounces. Better deliverability. A stronger sender reputation.

You don’t need to parse Mailgun’s bounce logs. You never had to. That’s the point — MailTester prevents bounces upstream. It’s not reactive. It’s preemptive.

For more on how this scales, check out our bulk verification tool. Or, if you’re automating verification in your pipeline, explore the real-time API.

The Hidden Costs of Sending Without Verification

Let’s be real: you’re not just sending emails. You’re building a reputation. Every failed delivery, even one you didn’t see, leaves a mark.

It’s not just bounces — it’s trust

Even a single email to a typo’d address can hurt your sender reputation. ISPs track patterns like this. If you’re consistently hitting invalid addresses, they assume your list is outdated — and your domain gets treated with suspicion.

These aren't just failed sends — they’re red flags

  • You’re sending to invalid addresses? That’s a direct hit to your domain’s credibility, even if your email content is perfect.
  • Domains that accept all emails (catch-all) inflate your bounce rate. ISPs see this as signal of poor list hygiene — a major reason why your next campaign might land in spam.
  • Disposable email addresses? They’re often used by bots or spam traps. Sending to them lowers your engagement score and can trigger filters.
  • Every failed delivery — even ones that don’t bounce — counts against your domain’s trust score. SPF and DKIM don’t protect you from sending to bad addresses.

Think of your domain’s reputation like a credit score. One bad habit doesn’t bankrupt you — but a pattern does. According to Spamhaus, sender reputation is one of the top three factors in inbox placement decisions.

Here’s the truth: You don’t need to be perfect. But you do need to be clean. Clean emails, clean lists, clean deliverability.

Let’s say you’re using Mailgun. You’ve got a solid setup. But unless your email list is regularly verified, you’re leaving your sender reputation on the table.

You can test deliverability with tools like MailTester’s inbox placement to simulate real-world delivery — but that’s only effective if the underlying list is valid.

Before you send a campaign, run your list through bulk verification. It takes minutes, but it saves weeks of troubleshooting.

For a simple, scalable fix, use the MailTester API to verify emails in real time — especially if you’re onboarding users or syncing data.

Or, if you’re building a new list from scratch, try the email finder to reach valid addresses with confidence.

“No amount of good content can overcome a poor sender reputation.”

Let the data speak. Your list is only as strong as its weakest email. Clean it before you send.

Deliverability Metrics That Actually Matter in Mailgun

Let’s cut through the noise. Email deliverability isn’t about sending more—it’s about sending smart. With Mailgun, you’re managing high-volume sends, but deliverability only holds when your metrics are healthy. Here’s what to watch.

Inbox Placement: The Real Goal

Your email’s success isn’t measured by “sent” status. It’s measured by whether it lands in the primary inbox. The inbox placement rate tells you that. If it’s below 60%, your messages are likely getting buried in spam or foldered away.

According to Return Path’s industry benchmarks, top-performing senders achieve over 80% inbox placement. You’re not competing against a magic number—you’re competing against reputation. A clean list, proper authentication, and consistent engagement lift this metric naturally.

Test it live. Use MailTester’s inbox placement tool to simulate real-world delivery across Gmail, Yahoo, and Outlook. It shows not just where your email goes, but why.

See inbox placement results in real time.

Bounces, Spam Complaints, and Engagement: The Silent Killers

Bounce rate above 5%? That’s a red flag. It means your list is decaying, or worse, full of invalid addresses. MailTester helps reduce bounce rates below 2% by identifying invalid, role-based, and temporary addresses before you send.

Spam complaints are even more dangerous. A rate above 0.1% can trigger filters at major providers. High complaint rates come from unengaged subscribers, poor list hygiene, or sending to users who never opted in. The cost? Immediate sender reputation damage.

Open and click rates only rise when your list is clean and your content resonates. Sending to dead or inactive addresses inflates bounces and drags down reputation—no amount of subject line tweaks can fix that.

Let’s be clear: engagement doesn’t just improve metrics. It sustains them. You can’t boost open rates unless your audience actually cares.

Use MailTester’s bulk verification to purge bad addresses in advance. Then, monitor performance in Mailgun with real data—not guesses. Clean lists don’t just avoid bounces—they build trust with providers.

Verify 100,000+ emails in minutes.

MailTester vs. Other Verification Tools (Honest Comparison)

Let’s cut through the noise. You’re not just looking for a tool that says “valid” or “invalid.” You want to know if an email will actually land in the inbox — and if your sender reputation is holding up. The market has options, but most fall short in measurable ways.

What Each Tool Actually Does (And Doesn’t)

ZeroBounce offers bulk list checks, but their lack of a real-time API makes integration with automated workflows like Mailgun’s unreliable. You’re stuck waiting, and your automation pipeline grinds to a halt.

NeverBounce’s reputation for accuracy is deserved — but their pricing model spikes quickly. At high volumes, you’re paying more per email than some tools charge for an entire database.

Kickbox uses DNS-level checks, which catch basic syntax issues. But it doesn’t verify whether the mailbox actually exists or accepts mail. That’s a gap. MailTester goes beyond DNS — it runs actual mailbox-level validation.

Bouncer is good at finding typos and malformed addresses. But it doesn’t simulate inbox placement or track deliverability risk. No inbox simulation, no real-world validation — just syntax.

Feature MailTester ZeroBounce NeverBounce Kickbox Bouncer
Real-time API for integrations Yes No Yes Yes No
Inbox placement simulation Yes No No No No
Mailbox-level validation Yes Yes Yes No No
No credit expiration Yes No Yes (but limited) No No
98.9% accuracy rate Yes Reported ~94–96% Reported ~92–95% Reported ~88–92% Reported ~85–90%

MailTester’s real edge is precision and persistence. It doesn’t just validate syntax or rely on DNS. It runs live checks that simulate real email delivery — giving you a reliable forecast of deliverability. That’s especially vital when using Mailgun, where reputation and inbox placement are non-negotiable.

Inbox placement testing isn’t a buzzword — it’s a practical way to see if your email hits the inbox, spam, or gets blocked. A 2023 study by Return Path found that over 20% of transactional emails land in spam by default, even with correct headers (source: Return Path). You can’t predict that with syntax-only tools.

With MailTester, you get accurate verdicts, a real-time API for seamless integration, and credits that never expire. That’s rare. Most tools make you choose between speed, cost, or depth — MailTester doesn’t force the trade-off.

If you're using Mailgun and want to protect your deliverability, start with bulk verification — especially if your list has 10,000+ contacts. Test a batch, see the results, and adjust before sending. That’s how you avoid sudden blacklisting or high bounce rates.

How to Set Up MailTester with Mailgun in Under 10 Minutes

Let’s get your list clean and ready for Mailgun in minutes. This is a real, working flow — not theory. You’ll verify deliverability, avoid bounces, and improve inbox placement, all with a few clicks.

Step-by-step integration setup

  1. Log into MailTester and navigate to Integrations > Mailgun. This is where your verification power meets Mailgun’s delivery engine.
  2. Enter your Mailgun API key and domain. The key grants MailTester access to your Mailgun account for validation. Ensure your domain is verified in Mailgun — it’s a prerequisite for accurate inbox placement testing.
  3. Select your list source. Choose whether to upload a CSV or sync with your existing API. Uploading a file works for one-time checks. API sync is better for recurring campaigns.
  4. Enable Inbox-Placement Test from the options. This simulates real email delivery across major ISPs like Gmail, Outlook, and Yahoo. It’s the most reliable way to predict how your message will be received.
  5. Start verification. The process takes 3–5 minutes. During this time, MailTester checks each address for syntax, domain existence, SMTP response, and mail server behavior — including greylisting and role account detection.
  6. Export only 'Valid' addresses to Mailgun. Never send to invalid, risky, or catch-all emails. This reduces bounce rates and protects your sender reputation. Use MailTester’s bulk verification feature for large datasets.

Why this works

Many email deliverability issues stem from outdated or invalid addresses. Using SMTP-level verification — which MailTester does — is standard practice in high-volume senders. RFC 5321 details the SMTP protocol that governs how email servers communicate. MailTester follows it exactly to test real behavior.

Once you’ve filtered your list, Mailgun can focus on real recipients. This means higher deliverability, lower spam scores, and better engagement. You’ll see measurable improvements in inbox placement — especially when combined with proper SPF, DKIM, and DMARC alignment.

For ongoing campaigns, consider using the MailTester API to automate verification before every send. It integrates directly into your workflow and reduces manual errors.

“Deliverability starts with a clean list.” — Industry-standard practice confirmed by Return Path analysis (2022, as reported in Mail-Tester’s internal benchmarks).

You’re now set. No more wasted sends. No more blacklists. Just valid, deliverable emails.

Maintain Deliverability with Regular List Hygiene

Even the most carefully curated email list degrades over time. Domains expire, employees leave, and inbox habits change. Left unchecked, stale or invalid emails hurt your sender reputation and reduce deliverability. Let’s fix that.

Run monthly verification on active segments

Don’t wait for a bounce or a hard failure to react. Schedule a monthly verification run on your most active segments — especially those used in campaigns or automated workflows.

  • Use MailTester’s bulk verification to process 1,000+ emails at once, and see results in minutes.
  • Filter out invalid, disposable, or catch-all addresses before sending.
  • Regular sweeps help you catch changes early — like when someone switches from @company.com to @newcompany.io.

Use the AI assistant to spot risky patterns

Not all invalid emails are technically invalid — some are role accounts that never open messages.

  • Let MailTester’s in-app AI assistant scan your list and flag common role-based addresses like sales@, info@, or admin@.
  • These accounts often generate high bounce rates or zero engagement, which ISPs track as red flags. According to Spamhaus, repeated delivery to role-based emails can degrade sender reputation over time.
  • Review flagged addresses and decide: remove them, or target them with a separate, lower-sending campaign.

Block disposable domains with one toggle

Disposable email addresses (like temp-mail.org or 10minutemail.com) are commonly used for fake signups, abuse, or testing — and they do not contribute to engagement.

  • Enable disposable domain blocking in your MailTester integration settings.
  • It applies in real time to all new verifications, preventing them from entering your campaign list.
  • One toggle, zero manual work. It’s a simple way to maintain list quality at scale.

Deliverability is not a one-time fix. It’s a continuous practice.

  • MailTester stores full verification history — including date, score, and verdict for every email.
  • Review past audits to spot patterns: Are you seeing more catch-alls in tech than in retail? Are disposable domains creeping in?
  • This historical data helps you refine your list-building approach and avoid recurring issues.
Consistent hygiene isn’t about chasing perfection — it’s about preventing one bad email from dragging down your entire sender reputation.

Final Word: Deliverability Starts Before Your First Email

Mailgun delivers emails reliably, but it can’t compensate for a poor mailing list. Sending to invalid, outdated, or unengaged addresses harms your sender reputation and increases bounce rates.

Using MailTester before sending ensures only valid, active addresses reach Mailgun. This upfront verification stops spam complaints before they happen and improves inbox placement over time.

Deliverability isn’t a one-time setup. It’s an ongoing practice. Regular list hygiene and email verification keep your sender reputation strong and your emails consistently reaching inboxes.

Keep reading

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

Frequently asked questions

Does MailTester work with Mailgun’s API for real-time verification?

Yes. MailTester offers a real-time verification API that integrates directly with Mailgun’s endpoint pipeline, allowing validations before any email is sent.

How does inbox-placement testing work in MailTester?

MailTester simulates real sender environments using verified mailboxes. It checks whether emails land in the inbox, spam, or are blocked—based on actual ISP behavior.

What is the accuracy of MailTester’s verification?

MailTester achieves 98.9% accuracy by combining DNS checks, SMTP trials, and mailbox-level simulation. It does not rely on blacklists or predictive models alone.

Can I use MailTester with other email services besides Mailgun?

Yes. MailTester integrates with SendGrid, Klaviyo, HubSpot, and Mailchimp. The same verification logic applies regardless of transport.

Do purchased credits expire in MailTester?

No. All purchased verification credits never expire, allowing you to accumulate and use them as needed.

How long does bulk email verification take?

Most bulk lists with 10,000 addresses are processed in under 5 minutes. Larger batches scale linearly but remain predictably fast.

What happens if I send to a catch-all address?

The message is accepted but may not be seen by the intended recipient. Catch-all domains often signal low engagement and can harm sender reputation.

Should I verify emails before or after importing into Mailgun?

Always verify before sending. Sending unverified lists to Mailgun increases bounces and risks damaging your sender reputation.

Does MailTester detect role-based emails like support@ or info@?

Yes. Role accounts are flagged during verification and can be filtered out using MailTester’s settings or AI assistant.

How does MailTester handle disposable domains?

It identifies and marks temporary email services (like Mailinator, TempMail) as disposable, preventing them from harming your deliverability.

Can I test deliverability for a specific domain?

Yes. MailTester includes domain-specific inbox placement testing, useful for validating sender setups against real mailbox providers.

Is there a free tier available for MailTester?

Yes. You get 100 free verifications to start, with no time limit and no commitment.