Why Do Mailgun and SendGrid Disagree on Your Email Deliverability?

You sent the same batch of emails through Mailgun and SendGrid. One says your messages landed in inboxes. The other says they’re stuck in spam or never reached recipients. You didn’t change anything. Why the mismatch?

Deliverability isn’t a single score—it’s a composite built from internal signals. Mailgun and SendGrid measure different things, weigh them differently, and apply different thresholds. The result? Two systems, two verdicts.

Think of it like two traffic cops at different intersections. One stops you for speeding by 1 mph. The other lets you pass. Both are right—within their own rules. The same happens with email delivery: different platforms track bounces, engagement, and sender reputation through distinct lenses.

Key takeaways

  • Mailgun and SendGrid use different internal metrics, timeframes, and spam thresholds, leading to conflicting deliverability verdicts.
  • One platform may flag a domain as risky due to historical engagement patterns, while the other reports clean status because of newer or lower threshold filters.
  • Reconciliation begins with understanding how each platform defines and tracks bounces, engagement, and sender reputation—factors that directly affect inbox placement.

The Root Cause: How SendGrid and Mailgun Measure Deliverability Differently

SendGrid and Mailgun report different deliverability outcomes because they prioritize different signals: SendGrid tracks whether messages land in inboxes based on real-time engagement, while Mailgun focuses on technical success, like SMTP error codes and DNS response times. Their spam filters don’t weigh header integrity, content patterns, or sender history the same way—meaning the same email can pass one system and fail the other. Let's break down why.

Engagement vs. Technical Success

SendGrid’s model leans into behavioral data. It measures inbox placement, open rates, and click-throughs to assess whether an email is truly deliverable. High engagement signals good reputation, which improves future delivery chances—this is how they define success.

Mailgun, by contrast, focuses on the delivery handshake: Did the server accept the message? Did it respond within a reasonable time? Did the DNS records resolve? A successful SMTP transaction doesn’t guarantee inbox delivery—but Mailgun treats it as a win.

Spam Filter Models Aren’t One Size Fits All

Even when both systems validate the same email, their spam filters assign different weights to red flags. SendGrid might ignore a minor header anomaly but penalize a low open rate. Mailgun may flag a mismatched Return-Path or a short, templated message body—even if it gets opened.

Header integrity—like SPF, DKIM, and DMARC alignment—is critical across both platforms, but their thresholds differ. For example, a missing or misaligned DKIM signature might trigger an immediate rejection in Mailgun but be ignored by SendGrid if engagement signals are strong.

Content patterns like excessive uppercase text, links in the first line, or repetitive phrasing can also trip different filters. While these traits correlate with spam, the point at which they cross the threshold varies between providers. You might be clean on one system and flagged on the other.

This isn’t a bug—it’s a feature. The models reflect how each platform interprets sender reputation. SendGrid’s approach mirrors real-world inbox placement, while Mailgun’s reflects backend delivery health. No single system is “right.”

Understanding this divergence helps you avoid panic when reports conflict. You don’t need to choose one. Instead, validate your list before sending with tools that catch problems early. Clean your list in bulk using real-time verification to avoid hard bounces, role accounts, and disposable domains—common causes of delivery issues across both platforms.

For a deeper check, test your email in real inboxes using inbox placement testing. It shows where your message lands across multiple providers, giving you clarity beyond raw deliverability metrics.

Mailgun’s Bounce Logic vs. SendGrid’s Engagement Scoring

Mailgun treats SMTP errors like “550 User unknown” as hard bounces immediately, marking the address as invalid. SendGrid often sees the same error as a soft bounce, especially if the domain accepts mail but the specific user doesn’t exist, so it keeps the address alive for engagement tracking. This mismatch means one system might purge an address while the other still sends to it — causing deliverability confusion and wasted sends.

How SMTP Errors Translate Differently Across Platforms

When Mailgun receives a 550 response from a recipient server, it assumes the mailbox is permanently unavailable and flags it as a hard bounce. That’s the standard interpretation in most email systems: the server says the user doesn’t exist, so the address is scrubbed.

SendGrid, however, applies a broader scoring model. It may interpret a 550 error as temporary if it detects the domain isn’t rejecting mail entirely—perhaps it’s accepting mail but rejecting invalid users at the mailbox level. This aligns with industry practices where soft bounces (e.g., full inbox, transient server issues) are treated differently from hard ones.

That difference in logic creates a real-world conflict. A user on a domain like example.com might not exist, but the domain does. Mailgun removes the address immediately. SendGrid, seeing the domain accepts mail, may still consider the address a candidate for engagement — perhaps waiting to test if it becomes active or assigning a low engagement score.

What This Means for List Health and Deliverability

The mismatch is not a bug, but a design distinction. Mailgun prioritizes list purity. SendGrid prioritizes engagement potential. Both are valid — but if you’re using both systems, you’ll get conflicting signals.

For instance, a list might be flagged as clean in SendGrid’s engagement reports but full of invalid addresses in Mailgun’s bounces. This makes reconciliation hard — unless you understand the root cause.

One way to resolve it is to validate the addresses at the source. Use a tool like bulk email list cleaning before sending. It applies consistent SMTP-level checks, MX validation, and catch-all detection — not just one platform’s interpretation. You’ll avoid conflicting reports by starting with a clean list that matches across systems.

For ongoing accuracy, pair that with real-time verification on new sign-ups. It gives you consistent feedback regardless of whether you're hitting Mailgun or SendGrid. Both platforms react differently to the same error — but your list shouldn’t.

This isn’t about choosing one system over another. It’s about knowing how each interprets failure. And building a process that doesn’t rely on any single system’s score. Use inbox placement testing to validate actual delivery, not just bounce codes.

Ultimately, your deliverability success depends on consistent data. Not conflicting reports. The only way to reconcile them is to verify addresses before they reach the engine — not after.

How Email List Validation Reveals Hidden Inconsistencies

You can reconcile conflicting deliverability reports between Mailgun and SendGrid by validating your list with a tool that checks real mail servers—not just platform-specific SMTP responses. Email List Validation performs real-time verification against active mail servers, exposing whether an email is truly live, a catch-all, or disposable. This independent check cuts through platform bias and reveals inconsistencies that automated systems miss.

Going Beyond SMTP: Validation Against Active Mail Servers

Most tools rely on basic SMTP responses—like “550 User unknown”—to judge an email’s validity. But that’s incomplete. A 550 might mean the user doesn’t exist, or it could mean the server doesn’t want to disclose that detail. That’s where real-time verification steps in. It simulates an actual inbox connection, probing the server’s behavior during actual delivery attempts. This method accounts for greylisting, rate limiting, and DNS-based filtering that platforms like Mailgun or SendGrid may filter out entirely.

For example, a server might temporarily reject a connection (a 4xx response) but later accept the same message. Platforms often treat this as a failure, while an active server check learns the difference. This matters when one provider reports 92% deliverability and another says 78%—the mismatch often comes down to how each interprets temporary failures.

The approach aligns with industry standards: RFC 5321 describes the SMTP protocol, but doesn’t mandate how to handle greylisting or throttling. That’s why relying on a single platform’s view of deliverability is risky. Independent validation fills that gap.

High Precision Across Edge Cases

Email List Validation’s 98.9% accuracy rate—based on real-world feedback and consistent test cycles—means it can reliably differentiate between valid, catch-all, and disposable domains. It doesn’t just say “valid” or “invalid.” It tells you when an email is technically live but unlikely to convert (a catch-all), or when it lives on a known disposable domain (like TempMail or GuerrillaMail).

With our bulk verification API, you can independently test every email in your list, regardless of whether it’s been flagged by Mailgun’s inbox placement tool or SendGrid’s postmaster reports. The API works in real time, so you can verify a list before sending, or check individual emails on demand. This gives you a neutral benchmark—untainted by the internal logic of any single platform.

Use it to audit your list before sending: clean your list at scale and resolve inconsistencies before they impact reputation. You’ll see why one platform said “deliverable” and another said “bounced”—because one was judging syntax, the other was judging behavior. The truth lies in the actual server response, not the platform’s filter.

A Step-by-Step Process to Reconcile Conflicting Reports

When Mailgun says an email delivered but SendGrid marks it as undeliverable, you need a neutral ground. Export both platforms’ delivery reports, align by recipient, and run a bulk verification with Email List Validation to determine each email’s true status—valid, invalid, catch-all, or risky. The system’s 98.9% accuracy gives you a definitive answer. Discrepancies? You’ll find them when a platform says “success” but validation says “invalid” — these are nearly always false positives.

  1. Export your delivery reports from both Mailgun and SendGrid. Ensure the export includes the recipient email and delivery status. Use consistent date ranges and remove duplicates. This gives you a single comparison set.
  2. Align both lists by email address. Use tools like Excel, Google Sheets, or a simple script to join the two reports on email. Only analyze emails present in both platforms — avoid noise from lists that don’t overlap.
  3. Run a bulk verification using Email List Validation. Upload the aligned list and let the tool process it. You’ll get real-time verdicts: valid, invalid, catch-all, or risky. This is the authoritative source — it checks DNS, MX, SMTP, and sender reputation in real time. Clean your list at scale with the most accurate results in the market.
  4. Compare each email’s validation result against both platforms’ delivery status. If validation says “invalid” but Mailgun says “delivered,” that’s likely a false positive. Some platforms report “delivered” even if the message bounced later or was caught by greylisting. This is why relying on platform scores is unreliable.
  5. Flag and document discrepancies. Keep a running sheet of emails where platform status and validation status conflict. Use the validated status as your truth. This removes guesswork and prevents future bounces or spam complaints.

Why Platform Scores Alone Are Misleading

Platforms like Mailgun and SendGrid focus on delivery signals — like SMTP receipt and queue status — not long-term deliverability. A server might accept a message and later discard it, or the recipient might have a role account that isn’t verified. This is where SMTP checks and real-time validation matter more than a simple “delivered” label.

According to RFC 5322, email delivery is confirmed only when the message reaches the recipient’s mailbox. Many platforms report “accepted” but not “delivered.” Validation tools go further by checking if the mailbox truly exists and can receive messages. That’s why a 98.9% validation accuracy rate beats platform-specific reporting.

Reclassify Your List on Verified Data

Once you’ve mapped all results, reclassify your entire list based on validation output. Retain only valid emails. Remove invalid, risky, and catch-all addresses. This ensures your sender reputation stays clean and inbox placement improves.

Why Platform-Only Data Leads to Over-Optimistic Send Rates

You're getting 95–97% delivery reports from SendGrid or Mailgun, but that number is misleading. These platforms count messages as “delivered” even when they land in catch-all or role-based inboxes—accounts like admin@ or sales@ that absorb mail without confirming receipt. A 97% delivery rate might include 14% of messages sent to invalid or non-existent destinations. This inflates your success metrics while masking poor list quality and real deliverability risks. Without deeper analysis, you’re optimizing on a false signal.

What Platforms Don’t Tell You About Deliverability

SendGrid’s dashboard reports “delivered” after an SMTP handshake—no matter whether the message reaches a real person. Many of these deliveries go to catch-all mailboxes, which accept email but never deliver it to a specific user. According to industry data, catch-all accounts can make up 10–20% of inbox traffic in high-volume sends, meaning your perceived success rate is inflated by volume, not engagement. A message sent to [email protected] may be technically “delivered,” but it’s not reaching the right audience.

Mailgun’s 95% success rate often skips over soft bounces that don’t fail immediately. These delays—sometimes a delay of several hours or days—means your platform might record an email as “delivered” even when the recipient server temporarily blocks it due to rate limiting, full inbox, or a server-side delay. Without tracking delayed failures, you miss early warning signs of sender reputation issues.

Why Relying on Dashboards Is a Technical Blind Spot

Both platforms rely on SMTP-level feedback, which is incomplete. They don’t verify if the email address is valid, if it belongs to a real person, or if the account is disposable, role-based, or inactive. The result? A high delivery rate that doesn’t reflect real user engagement. In practice, this leads to higher spam complaints, more bounces later on, and degraded sender reputation—especially when sending in bulk or to cold lists.

It’s like driving on a road that shows a green light at every intersection but doesn’t tell you whether the roads are clear. True deliverability isn’t about connection; it’s about reaching someone who will see, read, and act. You can’t trust a single platform’s metrics alone.

To test real inbox placement, not just SMTP success, use tools that simulate real user inboxes. We use inbox-placement testing to validate whether emails land in primary folders, not just spam or empty folders. This type of testing reveals the true health of your sends and identifies issues before they hurt deliverability.

For deeper list health checks, run a bulk verification to identify invalid, role-based, or disposable email addresses before sending. Clean your list at scale with tools that detect catch-alls and inactive accounts before they harm your reputation. Real-time verification can catch problems on signup—before they accumulate.

Clean your list before sending to ensure every message reaches a real person, not a server-level placeholder. You’ll get better engagement, lower bounce rates, and a stronger sender reputation—without over-relying on platform dashboard numbers.

The Role of Catch-All and Role Addresses in Deliverability Discrepancies

Mailgun may classify catch-all domains as valid because they accept all incoming emails—regardless of whether a real person receives them. SendGrid often flags these same addresses as high-risk due to poor engagement and low inbox placement, leading to conflicting deliverability reports. The root issue? Validity doesn’t equal engagement. You can verify an address exists, but that doesn’t mean it's effective for sending.

Catch-All Domains: The False Positive Problem

Catch-all domains are designed to capture any email sent to them, even to non-existent addresses. Mailgun, which prioritizes technical validity, sees these domains as “accepted”—because the server doesn’t bounce. But that acceptance doesn’t mean the message will be seen, read, or acted upon.

According to RFC 5321, a catch-all system is not a failure—it’s a design choice. But from a deliverability standpoint, it’s a trap. These addresses often belong to automated systems, spam traps, or role accounts like info@ or admin@. If you send to them, you’re not reaching a real user. You’re inflating your send volume without adding value—and that signals bad intent to mailbox providers.

Role Addresses: Silent Red Flags

Role accounts like sales@, support@, or info@ are typically monitored by bots or forwarded to teams. They rarely engage with messages, making them low-priority from a reputation standpoint. SendGrid’s algorithms flag these as high-risk because they’re rarely opened—but Mailgun may treat them as valid, especially if they don’t bounce.

Let’s be honest: you’re not trying to send to “admin” or “info.” You’re trying to reach people who’ll act. Sending to non-human endpoints is like sending mail to a dead letter office. It wastes bandwidth, degrades sender reputation, and increases spam complaints.

Real-time list validation can catch these discrepancies before you send. Tools like bulk email list cleaning analyze syntax, domain health, and engagement risk—including catch-all and role accounts—so you don’t get conflicting reports later. You’ll catch false positives early, improve inbox placement, and keep your sender reputation intact. It’s not about rejecting every role address—it’s about knowing which ones are worth including.

How to Test Inbox Placement Without Relying on Platform Claims

Stop trusting provider dashboards. Use real inbox testing to see exactly where your emails land—Primary, Social, or Spam—across Gmail, Outlook, and Yahoo. This reveals whether deliverability issues are from your list, sender reputation, or mailer configuration, not platform bias.

  1. Send test messages through Email List Validation’s inbox-placement test. This isn’t about bounce rates or internal scoring. It’s about sending to actual user inboxes and seeing where they end up. You’ll know for sure if your emails are landing in Primary, getting buried in Social, or worse—filtered to Spam.
  2. Use a verified sample set from your list. Don’t test arbitrary addresses. Pick 50–100 real, engaged recipients from across your segments. This gives you a realistic picture of delivery across different email clients and filtering behaviors.
  3. Test across multiple providers and inboxes. Gmail, Outlook, and Yahoo each use different spam filtering systems. A message that clears Gmail might land in Spam on Outlook. Test across all three to spot weak links in delivery. This is how real-world inbox placement is measured—and how you can validate consistency.
  4. Check timing and sender context. Deliverability isn't just about the email. It’s about your timing, domain reputation, and whether the recipient has interacted with your brand before. Use the test results to identify whether issues stem from list quality, content, or setup.
  5. Compare results to your Mailgun and SendGrid metrics. If your SendGrid dashboard says 99% delivery but your inbox test shows 70% landing in Spam, your internal metrics don’t reflect reality. This exposes the gap between provider claims and actual inbox presence.

Why Internal Platforms Can Mislead

Mailer platforms like Mailgun and SendGrid often report on delivery *to their infrastructure*, not *to real inboxes*. A message can “deliver” into SendGrid’s system but fail entirely in a user’s inbox. This mismatch is common and can mask real deliverability problems. The only truth comes from real inbox testing, as outlined in the RFC 5322 standards for email structure and routing, which emphasize end-user delivery, not internal logging.

Validate Consistency Across Your List

Run inbox tests on multiple list segments—active users, inactive users, new signups—and compare results. If only old subscribers show up in Spam while new ones land in Primary, you're likely dealing with outdated or low-engagement behavior patterns. Addressing this improves long-term reputation with providers like Gmail and Yahoo.

Use the inbox-placement test not just once, but periodically—after list cleaning, content changes, or domain switches—to isolate what’s actually working. This replaces guesswork with data that reflects real user experience.

Integrating Email List Validation with Your Existing Workflow

You can reconcile conflicting deliverability reports between Mailgun and SendGrid by validating your email list before sending—automatically cleaning invalid, risky, and disposable addresses using real-time verification tools, then integrating results directly into your email service provider workflows like SendGrid or Mailgun via API. This stops bounces before they happen and aligns your deliverability metrics across platforms.

Automate list cleaning at intake

  • Use the real-time verification API to check every address as it enters your system, reducing noise before it reaches your ESP.
  • Connect Email List Validation directly to SendGrid or Mailgun via their API to verify lists on upload—no manual steps, no guesswork.
  • Filter out catch-all, role-based, or disposable email patterns by flagging them during verification, which helps explain why one ESP marks a recipient as deliverable and the other doesn’t.
  • Prevent sending to addresses that fail DNS or SMTP checks—common sources of bounce discrepancies between platforms.

Use trusted integrations and intelligence

  • Sync with Mailchimp, HubSpot, Klaviyo, or SendGrid to clean data automatically before each campaign—ensuring both Mailgun and SendGrid see the same high-quality list.
  • Enable the in-app AI assistant to review verification results and explain why certain addresses were flagged as "risky" (e.g., greylisted, temporary, or high bounce risk), helping you reconcile conflicting sender reputation signals.
  • Run inbox placement tests via inbox placement testing to see how your messages perform across Mailgun, SendGrid, and major inboxes—providing a shared benchmark.
  • Use the bulk verification tool to process large datasets, then recheck deliverability on the cleaned list before sending.

Industry-standard practices like SPF, DKIM, and DMARC alignment are only effective when your list is clean. SPF validation checks fail more often with bad lists—even if setup is correct. By validating at the source, you reduce false delivery signals and standardize results across platforms.

“An email list with just 5% invalid addresses can reduce deliverability by up to 20%.” — industry observations from email deliverability best practices (e.g., Return Path, now Oracle Marketing Cloud)

When to Trust the Verification Result Over the Platform Report

If an email shows as delivered in Mailgun or SendGrid but your email validation tool flags it as invalid, trust the validation result. Platform reports reflect sending behavior, not address validity. Verification tools check DNS, mailbox responsiveness, and syntax—providing ground truth. When platforms disagree with validation, the validation result is the more reliable signal.

Delivery ≠ Validity

SendGrid may report an email as delivered, but that doesn’t mean the address is valid. Delivered means the message reached the recipient’s server—not that the mailbox exists or accepts mail. A single successful delivery can mask a non-existent or inactive inbox. For instance, a catch-all server will accept any email without checking if the user exists. This is common in corporate domains and high-volume mailing lists. Validation tools detect these catch-alls and mark them as invalid, even if the platform says delivered.

Bounces Don’t Always Mean Invalid

Mailgun marking an address as bounced doesn’t automatically mean it’s wrong. Bounces can stem from temporary issues like full inboxes, server timeouts, or greylisting—especially on busy mail servers. If validation says the address is valid, it’s likely a transient problem, not a permanent failure. A valid email might fail to receive a message due to server-side delays or filters. You should verify the address independently before removing it. Tools like bulk email list cleaning help you identify these edge cases with clarity.

Think of it this way: Mailgun and SendGrid report what happened during delivery. They don’t confirm whether the email address itself is correct or active. That’s the job of a verification tool. RFC 5321 and 5322 define how mail servers should behave, but delivery outcome is not a proxy for address validity. Industry data from RFC 5321 confirms that server responses alone—like 250 OK—tell us little about the actual existence of a user account.

Let’s say you’re cleaning a list and see conflicting signals. SendGrid says delivered, but validation says invalid. In that case, reject the platform’s claim. If Mailgun says bounce, but validation says valid, don’t assume failure—check for temporary conditions like greylisting or server lag. When in doubt, use real-time verification via an API to test individual addresses during onboarding or prior to sending. That’s the only way to separate signal from noise.

The Bottom Line: Reconcile Reports by Grounding in Validation, Not Assumption

Delivery platforms like Mailgun and SendGrid use internal scoring systems that are opaque and not directly comparable. Their bounce and engagement metrics reflect their own algorithms, not a universal truth about email validity.

Why Platform Reports Differ

Each platform evaluates inbox placement based on its own signals—historical engagement, spam complaints, engagement rate thresholds. These vary meaningfully across providers. Relying on one platform’s assessment alone risks misclassifying valid addresses or overlooking toxic ones.

  • Mailgun may flag an address as low engagement due to low open rates.
  • SendGrid might score the same address as high deliverability based on authentication alignment and past sender reputation.

These discrepancies don’t mean one is wrong. They mean both are incomplete.

Independent Verification Is the Only Reliable Anchor

To resolve conflicts, you need an external benchmark. Email List Validation tests each address using real-time SMTP checks, MX lookup, and domain policy verification—not engagement proxies.

It identifies invalid addresses, catch-alls, disposable domains, and role accounts—common sources of false positives in platform reports. With a 98.9% accuracy rate, it cleans your list before sending.

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 Mailgun and SendGrid disagree on the same email address?

Yes. They use different criteria and time windows to assess delivery, so one may report success while the other shows a bounce or risk flag.

Why does SendGrid mark a valid email as undeliverable?

It may treat soft bounces, catch-all domains, or role accounts as hard failures, especially if engagement signals are missing.

How do catch-all domains impact deliverability reports?

They increase reported delivery rates without real engagement, misleading platforms into thinking the list is healthy.

What is the most reliable way to test email deliverability?

Use independent inbox-placement tests with real mailboxes, not platform dashboards alone.

Does Email List Validation work with Mailgun and SendGrid?

Yes — it integrates with both platforms via API to validate lists before sending, improving accuracy and deliverability.

Can you trust SendGrid’s spam score over Mailgun’s?

No. Each platform uses proprietary spam models. Neither is universally accurate; verification is the only objective method.

What does 'invalid' mean in Email List Validation?

The address does not exist, the domain is unreachable, or the server rejects the email permanently.

How does Email List Validation detect role accounts?

It identifies patterns like admin@, info@, support@, and flags them as high-risk based on known behavior and usage.

Do disposable emails hurt sender reputation?

Yes — recipients who never open messages or mark them as spam damage reputation, even if delivery appears successful.

Can I use Email List Validation for list cleaning before sending?

Yes — bulk verification removes invalid, catch-all, role, and disposable addresses to improve list hygiene and sender reputation.

Does Email List Validation check for greylisting?

Yes — it detects delayed delivery responses and identifies addresses behind greylisting via server behavior analysis.

What happens if I don’t reconcile conflicting deliverability reports?

You risk sending to non-existent or unengaged addresses, which harms sender reputation and reduces inbox placement over time.