The Complete 2025 Guide to Mailgun Deliverability and Inbox Trust

You send a renewal reminder through Mailgun. It shows up as “delivered,” so you assume the customer received it.

A week later, they reached out frustrated because the renewal notice never reached their inbox. At this point, most teams think something went wrong with Mailgun. But that’s not really what happened. 

Mailgun did exactly what it’s designed to do: hand off the email to the receiving server. It doesn’t track inbox placement, engagement, or reputation health. And it’s not built to warn you when something starts slipping.

That’s the part many B2B teams miss. “Delivered” only confirms the message was accepted for processing, not that it reached the inbox.

Once Mailgun hands it off, the decision shifts entirely to Gmail, Outlook, and other providers. They evaluate your domain reputation, authentication, engagement patterns, and content — none of which Mailgun has visibility into.

This guide breaks down what Mailgun deliverability actually looks like, where the platform’s role ends, and where a dedicated deliverability layer like MailReach becomes essential if you want predictable inbox placement and early warning signals.

How Mailgun Deliverability Works 

Mailgun’s job is to send your email and hand it off to the receiving server. Once it gets a 250 OK, Mailgun marks the email as delivered and its visibility ends there. It has no insight into inbox placement, engagement, or filtering.

The flow is straightforward:

1. Your app sends the email to Mailgun.
2. Mailgun queues it, picks an IP, and applies suppression lists.
3. It connects to the recipient’s server and transfers the message.
4. The server accepts it (250 OK), and Mailgun stops tracking.

From this point, Gmail, Outlook, and Yahoo decide whether the message reaches the inbox, Promotions, or Spam. They base it on:

• Authentication alignment
• Domain reputation
• Engagement patterns
• Content quality
• Sending cadence

Transactional emails perform well because their patterns are stable and trusted. However, outreach and nurture emails don’t as new domains, volume spikes, and inconsistent engagement trigger filtering.

How to Monitor and Troubleshoot Mailgun Deliverability

Each email you send generates data that shows whether your infrastructure, sender reputation, or content is performing as intended. 

Monitoring and troubleshooting this data ensures your campaigns reach customers consistently, instead of being delayed or filtered out without warning.

1. Build your monitoring stack

The first step is to establish visibility into both delivery and inbox placement. 

Mailgun’s native tools provide delivery data, such as whether a message was accepted, rejected, or bounced. This view is useful for identifying infrastructure or DNS-related issues, but it doesn’t show where the email landed. 

That’s why you need a second layer like MailReach, which tests inbox placement and spam filtering across major providers such as Gmail and Outlook.

Together, these two systems provide a complete view of your email flow. Mailgun confirms that your servers are functioning correctly. MailReach verifies that your emails reach real recipients.

2. Establish a performance baseline

After activating your monitoring stack, establish baseline values for your key deliverability metrics. Consistent tracking enables early detection of issues before they disrupt customer communication or impact conversion performance.

• Delivery rate (≥ 98%)
  The delivery rate shows how many messages the receiving server accepted. When it drops, the cause is usually SPF/DKIM misalignment, IP throttling, or a decline in domain/IP reputation. In reputation-related cases, the receiving server may reject the message outright with bounce codes tied to “poor sender reputation,”.
  
  
• Bounce rate (< 3%)
  Hard bounces = invalid addresses.
  Soft bounces = temporary server issues.
  Rising bounces usually indicate list decay, which lowers reputation over time.
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• Spam complaints (< 0.1%)
  Even a small spike signals poor targeting or relevance and can quickly trigger filtering.

3. Maintain a regular review cadence

Ongoing analysis helps maintain a stable sender reputation and prevents gradual performance decline. Since each data point evolves at a different pace, schedule your reviews accordingly.

Run a weekly spam-placement test using MailReach to confirm your current inbox placement rate. Review Mailgun’s logs and suppression data monthly to identify any recurring bounces or complaint spikes. 

Audit your SPF, DKIM, and DMARC records quarterly to confirm they still align after DNS updates or hosting changes.

This cadence ensures your sending infrastructure remains predictable. If these reviews are skipped, issues such as an expired DKIM key or a stale list segment, can quietly damage your sender reputation for months before you notice.

4. Run targeted diagnostics

When performance metrics shift, isolate the cause quickly.

• Delivery rate drops: Common causes are SPF/DKIM misalignment or temporary blocks. Recheck authentication and verify your sending IP isn’t listed on a blacklist.
• Bounce rate rises: Remove invalid addresses immediately. Leaving them in rotation damages domain trust.
• Spam complaints increase: Typically caused by irrelevant targeting or sending too frequently. Tighten segmentation and adjust messaging.
• Inbox placement falls while other metrics look normal: This usually signals domain reputation decay. Lower volume temporarily and use MailReach warm-up to rebuild positive engagement patterns.
• Open rates decline: Review content structure: missing plain text, broken links, or messy HTML often trigger filters.

Note: Domains left inactive for more than three weeks begin to lose engagement history, which can cause mailbox providers to treat them as new. 

5. Automate early alerts

The most effective deliverability systems detect problems before recipients notice them. Connect Mailgun webhooks to alert your team whenever bounce or complaint rates spike and combine that with MailReach’s inbox placement notifications in Slack or email.

These alerts prevent cascading failures. A small DNS error, sudden volume spike, or content block can be fixed within hours instead of being discovered weeks later, after campaign damage has already occurred

When Mailgun’s delivery signals and MailReach’s reputation tracking work together, you create a closed feedback loop. 

Steps to Configure Mailgun for Maximum Deliverability

Deliverability starts with proper domain verification and authentication in Mailgun. Without this foundation, mailbox providers won’t trust your traffic.

Follow these Mailgun configuration steps so that mailbox providers recognize your domain as a trusted sender. 

Step 1: Verify domain ownership

Add your sending domain to Mailgun. You find this option in Mailgun › Domains › Add New Domain. 

When Mailgun displays TXT and CNAME records, add those records to your DNS provider (or DNS control panel). Once verified, Mailgun links your account to the domain, proving ownership.

Keep in mind that until this step is complete, mailbox providers treat your messages as unverified traffic

Step 2: Set up SPF (Sender Policy Framework)

Add the following TXT record to DNS:

“v=spf1 include:mailgun.org ~all”

This tells mailbox providers that Mailgun is authorized to send for your domain. If you use other platforms to send email, make sure they’re included in the same SPF record instead of creating separate ones.

Step 3: Configure DKIM (DomainKeys Identified Mail)

In Mailgun, generate two CNAME entries: 

1. k1._domainkey
2. k2._domainkey

After Mailgun generates the CNAME entries, add them to your DNS provider and enable DKIM signing in the Mailgun dashboard.

Without a valid DKIM signature, mailbox providers treat your messages as less trustworthy, which increases the chances of filtering over time.

Step 4: Establish a DMARC baseline

DMARC doesn’t improve inbox placement, but it helps ensure your SPF and DKIM results align with your From domain. Start with p=none to collect alignment data before moving to quarantine or reject.

v=DMARC1; p=none; rua=mailto:dmarc-reports@yourdomain.com

This is intentional because it lets you gather aggregate reports while you confirm everything is aligned. Once results are consistently good, tighten the policy to quarantine or reject. 

Step 5: Use a custom tracking domain

By default, Mailgun tracks opens and clicks through mailgun.org links. Create a branded tracking subdomain like email.yourcompany.com and use that instead.

This isolates your reputation from other Mailgun senders and keeps your tracking consistent with your own domain.

Step 6: Validate your setup

After DNS propagation, send a test message or use an email spam tester.

Check the headers for:

• spf=pass  
• dkim=pass  
• dmarc=pass

If any field shows “softfail” or “neutral,” fix the configuration before you scale volume.

Once all checks pass, you have a clean authentication baseline, and mailbox providers can verify your identity correctly.

MailReach vs. Mailgun: Understanding When to Layer Third-Party Email Tools

Mailgun provides a strong technical foundation with reliable delivery, bounce management, and IP handling. However, its native tools are designed for stability rather than granular control. Once you understand where Mailgun’s capabilities end, you can determine when adding a dedicated deliverability layer like MailReach becomes valuable.

Mailgun’s built-ins

Mailgun provides:

• Automated IP warm-up: Scales send volume gradually, but it’s volume-based rather than engagement-based. It doesn’t use open or reply signals (recipient engagement) and therefore can’t build a true reputation curve with inbox providers.
  
  
• Analytics: Reports show delivery success (the 250 OK hand-off), but they do not indicate inbox placement. Many users interpret “delivered” to mean “inbox placement,” which can lead to false confidence.
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• Suppression lists: Suppression lists are reactive. They block addresses only after a bounce or complaint occurs. While this protects Mailgun’s IPs, it doesn’t prevent a sender’s deliverability problems from originating.

Mailgun’s system reliably delivers messages to recipient servers, but it does not guarantee inbox placement 

Why add a third-party deliverability layer

Dedicated deliverability tools extend visibility and control beyond Mailgun’s infrastructure:

• Warm-up networks that mimic real human engagement. They send and reply to your emails from genuine inboxes, signaling positive engagement to mailbox provider algorithms.
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• Spam testing across hundreds of inboxes. You can see how Gmail, Outlook, and Yahoo mail classify your messages before you launch a full campaign..
  
• Early-warning systems for domain reputation. Real-time reputation and placement monitoring detects reputation decay weeks before it impacts conversions.

MailReach: built for precision control

MailReach integrates directly with Mailgun accounts and extends them with three layers of intelligence:

• Co-Pilot diagnostic engine: This analyzes your sending activity to identify practices that may be harming deliverability. It highlights the specific ISP or configuration factor involved and provides clear, prioritized recommendations to fix the issue.
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• Alerts for deliverability drops: MailReach monitors inbox placement and reputation signals across your sending domains and alerts you when deliverability declines. This gives you immediate visibility into shifts that Mailgun’s reporting does not surface, allowing you to respond before performance drops further.
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• Real-inbox network: Unlike tools that rely on SMTP accounts, MailReach interacts with actual Google Workspace and Microsoft 365 inboxes, the same environments where most B2B messages land.

Together, these systems automate warm-up and spam testing through authentic inbox interactions and provide actionable diagnostics, including: which provider is filtering your messages, what triggered the issue, and recommended recovery steps.

Decision framework

For B2C high-volume marketing, Mailgun’s native setup is usually sufficient, so a deliverability layer is not always necessary.

If you use Mailgun for B2B outreach, prospecting, onboarding sequences, demo follow-ups, or renewal reminders, adding MailReach gives you visibility that Mailgun alone cannot provide, particularly inbox placement, reputation behavior, and engagement patterns across business inboxes like Google Workspace and Microsoft 365. 

Keeping Your Mailgun Deliverability Strong

When your authentication is properly configured, your sending cadence is consistent, and your messages are genuinely valuable, inbox placement becomes consistent and predictable.

The real differentiators are consistency and discipline. Deliverability isn’t something you fix once and forget. It’s something you continually maintain. Track your signals, renew engagement, and warm up inactive domains before increasing volume again.

If you’re ready to start improving today:

• Run an email spam test to determine inbox placement for your Mailgun emails.
  
• If your domain has been inactive or underperforming, launch an automated email warm-up with MailReach to gradually rebuild trust.
  

When used together, Mailgun and MailReach provide both the delivery performance and diagnostic insight you need to maintain confident control over your inbox placement.

Sign up for MailReach now.