Test Email Deliverability Before Switching

Why testing deliverability is mandatory before switching email providers

You’ve spent weeks finalizing the switch from SendGrid to Mailgun. Your templates are reconfigured, your DKIM keys are set. But what if your next campaign lands in spam folders—despite perfect syntax and clean content?

Switching providers isn’t just a backend migration. It’s a shift in sender reputation, IP warmup behavior, and how your emails are filtered. Even small mismatches in infrastructure can derail inbox placement. Without testing, you’re trusting a new system with no proof it will work.

That’s why testing deliverability before switching is mandatory. You’re not just copying a list—you’re validating whether your domain, content, and recipients are ready for Mailgun’s engine. Tools like Email List Validation let you simulate real user inboxes and identify issues before they hit your audience.

Key takeaways

Switching from SendGrid to Mailgun changes how your emails are evaluated by inbox providers, requiring proactive deliverability testing.
Without testing, even valid emails may fail to reach inboxes due to sender reputation differences or content filtering.
Email List Validation’s inbox placement tests confirm whether your domain and message content meet Mailgun’s infrastructure requirements.

How to test email deliverability before switching from SendGrid to Mailgun

You should send test emails from your Mailgun domain to inboxes across Gmail, Outlook, Yahoo, and Apple Mail using a verified list with known outcomes—valid, invalid, catch-all, and disposable addresses—to check for bounces, spam markings, delays, or missing messages. Compare these results against your historical SendGrid performance to isolate Mailgun-specific issues. Then verify your SPF, DKIM, and DMARC records are correctly published before moving forward.

Step-by-step deliverability test process

Build a diverse test list with known email statuses: valid, invalid, catch-all, and disposable addresses. Use a tool like bulk email list cleaning to verify and categorize your list before sending.
Send test emails from Mailgun using your verified domain. Use real sender addresses and subject lines that mirror your usual campaigns to simulate actual conditions.
Check results across major inboxes—Gmail, Outlook, Yahoo, and Apple Mail. Look for delivery failures, spam folder placement, delayed arrivals, or missing messages. A failure in one inbox type may signal a routing or authentication issue.
Compare your results with SendGrid benchmarks—if you have historical data. Differences in delivery rates, spam scores, or bounce sources help identify Mailgun-specific issues rather than broader email trends.
Verify DNS records are properly configured. Ensure SPF includes Mailgun’s servers, DKIM is published with a valid key, and DMARC is set with a policy (none, quarantine, or reject). Use tools like MXToolbox or RFC 7483 to check alignment and reachability.
Monitor feedback loops and spam reports. If you're enrolled in any, check if Mailgun is receiving them. Absence can delay threat detection.

Why this works

Testing with a known, diverse list gives you measurable, repeatable feedback. A catch-all address confirming receipt but rejecting the email reveals delivery vs. acceptance. A valid but bounced address exposes a configuration fault. When you isolate these patterns early, you're not guessing—you're seeing real-world performance.

Authentication is the foundation—without properly published SPF, DKIM, and DMARC, even well-crafted emails from Mailgun will fail. This check ensures you're not just sending emails but sending them with legitimacy.

The hidden cost of skipping deliverability tests

You risk missing critical issues in your list—like old spam traps, invalid addresses, or low reputation signals—before migrating from SendGrid to Mailgun. A single poor campaign on a new domain can trigger blacklisting and take weeks to recover from, even if your messages are legitimate. Fixing deliverability problems after the switch is far more expensive than testing beforehand.

Even a small drop in inbox placement can cost you

Even a 1% decline in inbox placement on a 100,000-email campaign means 1,000 fewer messages delivered. That’s not a rounding error—it’s lost engagement, revenue, or conversion potential. When switching providers, the risk of a drop multiplies because you’re starting fresh with a new domain’s reputation.

Many teams assume email delivery is “just work” once set up. But deliverability is dynamic. It depends on sender reputation, list health, engagement signals, and infrastructure. Without testing, you’re flying blind.

Spam traps and stale lists are the silent killers

Stale email lists often include addresses that were abandoned years ago—and those are prime spam trap territory. Sending to these addresses can trigger blacklists, especially when moving to a new provider like Mailgun with no prior sending history.

Spam traps are not just a theoretical risk. The Spamhaus Project tracks active spam traps and publishes real-time data on known malicious infrastructure. You can’t control every trap, but you can eliminate known ones from your list. That starts with verification.

Consider this: a single spam trap hit might not get you blocked immediately. But repeated exposure, especially on new domains, can signal poor list hygiene to email providers. That signal can linger in reputation systems like SenderScore or Barracuda’s reputation database for weeks.

Before you switch from SendGrid to Mailgun, clean your list. Use real-time email validation to flag invalid, risky, or catch-all addresses. Tools like bulk email list cleaning can identify and remove these risks before they hurt your deliverability. Then run inbox placement tests to simulate actual delivery—on Gmail, Outlook, and Yahoo—so you know exactly what to expect once you move.

Don’t assume Mailgun’s infrastructure will save you. Your list’s health is still the bottleneck. A clean list with a strong sender reputation gives you better results, faster.

How to simulate your new Mailgun setup without going live

You can test your Mailgun setup’s inbox placement accuracy before migrating by sending real emails from your Mailgun domain to a sample of your list using Email List Validation. This tool delivers actual messages through Mailgun’s infrastructure and reports delivery results—whether each email reached the inbox, spam folder, or failed—based on real recipient inboxes, not just sender reputation scores or simulated filters. The results reflect actual filtering behavior across major providers like Gmail, Outlook, and Yahoo, giving you actionable insight before you switch.

Send real emails from your new domain while it’s still in setup mode

Before you switch your entire sending stack, send test emails from your Mailgun domain to 200–500 real addresses from your list. This is the only way to know if your new domain will be blocked, flagged, or filtered—especially if it’s new or has a weak sending history. Email List Validation does this at scale, sending from Mailgun’s infrastructure, so you’re testing with your actual outbound path, not a sanitized version.

See the real-time delivery outcome per inbox

Unlike tools that report only “good” or “bad” based on syntax or domain reputation, Email List Validation shows you exactly where each email landed: inbox, spam, or bounced. For example, one email might reach the inbox, another might land in spam due to a new IP blocklist, and another might fail due to a catch-all domain. This detail helps you identify if Mailgun’s IP or domain setup is already triggering filters.

Real deliverability isn’t just about reputation—it’s about what actually happens when a message lands in a specific user’s inbox. The industry standard for email validation is to test with real email clients and real inboxes, which is why Mailgun’s own documentation recommends warming up domains and testing with actual recipients before full-scale use.

You’re not just checking if emails go out—you’re confirming they arrive where they’re meant to. This step prevents surprise blacklists, failed campaigns, and wasted send volume after migration. Use this preview to adjust your DNS settings, warm up your IP through gradual sending, or correct list hygiene issues before you go live.

To test inbox placement with your Mailgun domain, try our inbox placement tool and send real emails from your new setup to real inboxes: test inbox delivery without sending to your whole list. Start with your top 500 engaged contacts to validate domain health, then scale up. It’s the most reliable signal you’ll get before migration.

What to check in your domain setup before switch

Before switching from SendGrid to Mailgun, verify your domain setup to avoid deliverability issues. Ensure SPF, DKIM, and DMARC are correctly configured, reverse DNS is set, and your sending IPs are not blocked. These steps are standard industry practice and align with guidelines from Spamhaus and the IETF’s RFC 5321. You can spot-check your setup using tools like MxToolbox or Email List Validation’s inbox placement test.

Core DNS and authentication settings

Confirm your Mailgun domain has a valid SPF record allowing Mailgun's IPs. Avoid overlapping records—only one SPF per domain is effective. Use RFC 7208 for standards-compliant syntax.
Set up DKIM via Mailgun and publish the key in DNS. Use a consistent selector (like "mailgun") and ensure the TXT record is correct—mistakes break alignment and hurt sender reputation.
Enable DMARC with a policy of p=none during testing. This won’t reject emails but allows you to collect reports and monitor domain exposure. Check reports weekly using a DMARC analyzer.

Infrastructure and IP alignment

Verify that Mailgun’s IPs have proper reverse DNS (PTR) records tied to your sending domain. Without this, some ISPs may reject your messages.
Check if your domain has been flagged on public blocklists like Spamhaus. Use Spamhaus to confirm your domain and IP are clean before switching.
Test your domain’s overall reputation. Tools like Mail-Tester or Email List Validation’s inbox-placement service can simulate delivery and surface risks.

Even a single misconfigured DNS record can cause inbox placement to drop below 70%.

Use real-time verification to clean your list before migration

You should verify every email address in your list using Email List Validation’s real-time API or bulk verification before switching from SendGrid to Mailgun. This filters out invalid, catch-all, or risky addresses that can trigger spam filters, damage your sender reputation, and reduce inbox placement. With 98.9% accuracy, you keep valid contacts while removing those that would harm your new platform’s performance.

Check for problematic emails early

Run your list through Email List Validation’s bulk verification or real-time API to catch invalid formats, non-existent domains, or disposable email addresses. These are common in unclean lists and can cause hard bounces, which hurt deliverability on any new provider—Mailgun included. Let’s not assume your list is clean just because it’s been used before.

Disposable domains and role accounts (like info@, support@, sales@) typically don’t open emails and don’t represent real users. While they may not technically bounce, they dilute engagement metrics and can flag your sender as a spam source. Tools like Email List Validation specifically detect these, so you can exclude them before migration.

Catch-all emails—accounts that accept mail for any address on a domain—also degrade sender reputation. Receiving replies or failed delivery notifications from them falsely indicates high engagement or delivery problems. Email List Validation identifies these so you can remove them, preventing confusion during deliverability testing.

Why 98.9% accuracy matters: it means you’re not over-cleaning. You’re not removing real users by mistake. This precision helps you preserve your core audience while still improving list health. That balance is essential before switching mail providers, where deliverability starts from day one.

For more detail on deliverability testing and how to verify sender reputation, see inbox placement testing—it shows how your messages land in real inboxes before and after migration. You can also use the real-time API to integrate verification into your signup or send workflows, ensuring ongoing list hygiene.

Industry best practices—like those outlined in RFC 5321—stress validating emails early. Preventing bad addresses from entering your system is more effective than handling bounces later. The cleaner your list, the faster Mailgun can build sender reputation.

How list hygiene affects deliverability on Mailgun

You can’t guarantee inbox placement on Mailgun if your list contains 15% invalid or inactive addresses—many sending systems, including Mailgun’s, automatically flag or reject lists with such high bounce rates during initial delivery. A high volume of invalid emails early on signals poor list quality, which can trigger throttling, limit sending volume, or even block future messages. That’s why cleaning your list before switching is not optional—it’s foundational.

Why list quality matters from day one

Mailgun treats new domains with caution. If your first 30 days of sending generate more than a few hundred bounces, their filtering systems will likely throttle your sending rate or pause delivery entirely. This isn’t an extreme case—it’s standard behavior for email providers to protect their reputation and inbox quality.

Even one bad email can harm your sender reputation, which is why a list full of old, incorrect, or dormant addresses increases the risk of delivery failure. A clean list improves your chances of landing in the inbox. According to industry data from Return Path, emails from new domains with clean lists see up to 30% higher inbox placement than those with unverified addresses.

How hygiene protects your sender reputation

Spam complaints and bounces directly impact your sender reputation, which Mailgun monitors closely. Even a single abuse complaint can trigger a review. A clean list reduces both bounces and complaints—key signals that your brand is trustworthy and relevant.

It’s not just about avoiding blacklists. It’s about building a sustainable sending profile. The higher your sender reputation, the more Mailgun trusts you to send in volume without penalty. That’s why maintaining a healthy list isn’t a one-time task—it’s part of ongoing deliverability management.

Let’s be clear: you can use all the right tools, set up DMARC, and follow best practices—but if your list is full of dead ends, your messages won’t land. The fix isn’t more emails. It’s fewer bad ones.

If you’re planning to switch from SendGrid to Mailgun, don’t assume your list will transfer cleanly. Use a tool like bulk email list cleaning to identify risks before they impact your deliverability. Real-time verification tools can also catch issues before they hit your inbox.

When to start warming up your Mailgun domain

You should begin warming up your Mailgun domain 7 to 14 days before sending your first bulk email campaign. Start with 50 to 100 emails daily to a known-valid list, gradually increasing volume while watching delivery rates and spam complaints. Use real inbox tests—like those from Email on Acid or Litmus—to verify performance; internal tools alone won’t show what inbox filters are actually doing.

Warm-up process: a step-by-step guide

Identify a clean, permission-based list—only send to subscribers who’ve opted in. Sending to invalid or unengaged addresses damages sender reputation from day one.
Send 50–100 messages daily to that list for the first 3–5 days. Keep content consistent and relevant. This low volume signals to ISPs that you’re not a spammer.
Monitor delivery and inbox placement using a tool like inbox placement testing. Real-time results help you spot if emails are landing in spam folders or being blocked entirely.
Gradually increase volume—add 50–100 emails per day every 2–3 days, up to your intended send volume. Slow scaling prevents reputation spikes.
Track spam complaints and bounces. A complaint rate above 0.1% is a red flag; a bounce rate above 2% is concerning. Both can trigger ISP blocks.
Verify list quality beforehand using bulk email list cleaning to remove invalid, dormant, or disposable addresses—this improves warm-up efficiency.

Why real inboxes matter more than internal tools

Internal testing tools can’t replicate how Gmail, Outlook, or Apple Mail decide what goes to the inbox. You need to see whether your actual emails land where they should. According to Spamhaus, even small sender reputation spikes due to poor list hygiene can trigger filtering. Your warm-up success must be measured in real user inboxes, not just server logs. Tools like real-time email verification help you clean your list before sending, so your warm-up isn’t undermined by dead or catch-all addresses.

Why inbox placement testing beats reputation scoring

Reputation scores like Sender Score are useful guides, but they’re estimates, not guarantees. An email can pass with flying colors on a reputation metric yet still land in spam because content, sending volume, or alignment with inbox filters triggers delivery blocks. Only inbox placement testing with real mail clients—like Gmail, Outlook, or Apple Mail—shows what actually happens. You’re not guessing. You’re seeing.

Reputation scores measure history, not outcome

Sender reputation systems track past behavior: sending patterns, bounce rates, and engagement. They’re built on historical data and predictive models. But they don’t account for how a single email might be marked spam today due to subject line quirks, image-heavy content, or even a mismatched sender name. A high score doesn’t ensure inbox delivery; it just means you’ve stayed out of trouble so far.

As the Internet Society notes in RFC 5321, the SMTP protocol doesn’t define what “trusted” means—only how delivery attempts are made. That means reputation scoring is a proxy, not a direct signal. It’s like checking tire tread depth to guess how well a car drives in rain. It helps. But you still need to test the brakes, windshield wipers, and actual driving conditions.

Inbox placement is the real test

Let’s be clear: only real-world inbox placement testing reveals the truth. This means sending test emails through your chosen provider—Mailgun, in your case—and having them evaluated across actual email clients and filtering systems, including spam thresholds. It shows the outcome: delivered, spam, or blocked.

Many tools offer “reputation” insights, but they’re models. Inbox placement testing, like the kind our platform enables, runs actual trials through real inboxes. It captures how your message lands—not how a system predicts it might. You’ll see which clients tag your email as spam, why, and what you can fix. The difference is measurable, not statistical.

For example, a campaign might score well on Sender Score but end up in spam folders across 90% of Gmail and Outlook tests. That’s not a flaw in the scoring system. It’s a flaw in how the content and format interact with modern filtering. You can’t catch that with reputation alone.

If you're switching from SendGrid to Mailgun, don’t just assume your reputation will carry over. Verify deliverability with real tests first. Use inbox placement testing to see how your emails arrive—from the first bounce to the final inbox.

See how your emails land in real inboxes with our inbox placement test: test your delivery performance before switching.

Final checklist before switching from SendGrid to Mailgun

Switching email providers isn’t just a technical change—it’s a deliverability reset. Without careful preparation, your message might never reach the inbox.

Even a small misstep can trigger bounces, spam complaints, or domain blacklisting. A clean, verified list and confirmed authentication are non-negotiable.

Final checklist

Run inbox placement tests via Email List Validation on a sample list to gauge real-world delivery success.
Clean and verify your entire list—remove invalid, disposable, and role-based email addresses to protect sender reputation.
Confirm SPF, DKIM, and DMARC are properly configured and aligned across all domains and subdomains.
Start domain warm-up at least 7 days prior to full migration to build trust with recipient mail servers.
Monitor bounces, spam markings, and open rates in the first 72 hours after launch to catch issues early.

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

Can I test deliverability with Mailgun before fully switching?

Yes. Send test emails from your Mailgun domain to real inboxes using a verified list to check inbox placement, spam filtering, and bounce rates.

How long does it take to warm up a Mailgun domain?

A minimum of 7 days is recommended, with gradual volume increases from 50 to 100 emails per day initially.

Does Email List Validation work with Mailgun?

Yes. The tool can test inbox placement from your Mailgun domain and integrate with SendGrid, HubSpot, Klaviyo, and Mailchimp.

What happens if my list has too many invalid email addresses?

High invalid rates lead to bounces, which harm sender reputation and can cause throttling or domain blacklisting.

Can I use fake email addresses to test inbox placement?

No. Testing with disposable or fake addresses gives misleading results. Use active, real inboxes for accurate placement data.

Why do some emails land in spam even with proper setup?

Spam filters evaluate content, sending frequency, list hygiene, and engagement. Even valid setup can fail if email behavior triggers heuristics.

Do I need to set up all three (SPF, DKIM, DMARC) on Mailgun?

Yes. All three are required for strong authentication and reliable delivery. Without them, many providers will reject or flag your emails.

Can Email List Validation detect spam traps?

Not directly, but by identifying invalid or inactive addresses, it helps reduce exposure to traps commonly found in outdated lists.

What’s the cost of switching providers without testing?

The cost can include lost conversions, blacklisting, and months of recovery time due to poor sender reputation.

How many free verifications does Email List Validation offer?

You get 100 free verifications to start, and any purchased credits never expire.

Does Email List Validation test deliverability from other providers?

Yes. It tests deliverability from your current platform (e.g., SendGrid) and your target platform (e.g., Mailgun) to compare performance.

Is inbox placement testing reliable on a test list?

Yes, when using real inboxes and verified domains. Results reflect actual receiving behavior in popular email clients.