Resolving Conflicts in Email Validation Outcomes Between Mailgun and SendGrid

Why Do Mailgun and SendGrid Disagree on Email Validity?

You send the same email list through Mailgun and SendGrid. One says every address is valid. The other flags half as disposable, invalid, or at risk. You’re not losing your mind — you’re seeing a real divergence in how these platforms judge email validity.

They don’t use the same rules. Mailgun checks if the server will accept the email during an SMTP handshake. SendGrid focuses more on DNS records, domain reputation, and role accounts. These different approaches can produce opposite verdicts — even for the same address.

This isn’t just a technical quibble. It directly impacts your deliverability, sender reputation, and campaign performance. When validation disagrees, you’re left guessing which system to trust.

Key takeaways

• Mailgun’s validation relies on SMTP handshake responses, making it sensitive to temporary server delays like greylisting.
• SendGrid uses DNS checks and role account detection, which reduces false positives on certain addresses but may miss catch-all domains.
• Disagreement between platforms is expected and stems from differing validation priorities — not error or flaw.

The Core Problem: Validation Conflicts in Practice

You’re verifying a list and find the same email passes Mailgun’s SMTP check but fails SendGrid’s DNS validation. Another address bounces on SendGrid but is marked valid in Mailgun due to a catch-all domain. These inconsistencies aren’t bugs—they’re built into how different providers weigh risk and interpret standards. The result? A fragmented view of list health, unreliable deliverability forecasts, and wasted effort on emails that may never land in inboxes.

Why the Same Email Can Get Different Verdicts

Mailgun focuses on real-time SMTP handshake success—can it connect, send, and receive a test message? If yes, it’s “valid.” SendGrid leans more on DNS-level checks: does the domain have valid MX records? Are SPF/DKIM configured? A valid SMTP handshake doesn’t guarantee DNS hygiene, and that’s where the split happens.

Then there’s catch-all domains—those that accept any email address for a given domain. If your list includes [email protected] and the domain accepts all incoming mail, Mailgun may return “valid” because the connection worked. SendGrid, knowing the address is not necessarily real, often flags it as “risky” or “invalid.” This gap between connection and actual address validity is where trust breaks down.

How Conflicting Results Impact Deliverability Planning

When one tool says an address is valid and another says it’s risky, you’re left guessing. Do you send to it? Ignore it? The answer depends on your risk tolerance—and your sender reputation.

Using only one tool introduces bias. Relying solely on SMTP checks like Mailgun risks including addresses from catch-all domains, which inflate bounce rates and hurt sender reputation over time. Trusting only DNS analysis risks dropping real emails that use flexible delivery rules (e.g., some enterprise domains).

That’s why a third-party, independent verification layer is essential. Tools like Emaillistchecker.io’s bulk verification use a combination of real-time SMTP, DNS, and pattern analysis to surface these inconsistencies, giving you a clearer, more reliable list health score.

Understanding the trade-offs between validation methods is critical. The IETF’s RFC 5321 describes SMTP behavior, and RFC 5322 defines email format—but neither mandates how providers assess “validity.” This gap allows for divergence in outcome based on implementation.

Let’s be honest: no single tool has the full picture. But having a consistent, cross-checked system—like Emaillistchecker.io’s 98.9% accuracy approach—lets you resolve these conflicts before they become deliverability or compliance issues.

Ultimately, your success isn’t about which tool is “right,” but about recognizing that every provider has a different lens. You need a neutral ground to resolve the differences.

What Each Verdict Really Means: Valid, Invalid, Catch-All, Risky

You’re not just getting “valid” or “invalid”—you’re getting a real-time breakdown of whether an email is actually deliverable, a placeholder for spam traps, or a role-based address that’s likely to bounce. Let’s break down what each verdict truly signifies, so you’re not misled by conflicting results between Mailgun and SendGrid.

Understanding the Judgments

When two tools disagree on an email address, it’s rarely about which one is “right”—it’s about how deeply they look. The most accurate systems combine DNS checks, SMTP interaction, and behavioral data.

Why Results Differ Between Mailgun and SendGrid

Mailgun and SendGrid both verify at the SMTP level, but they may use different fallback logic or caching. One might flag a catch-all address as valid; another might mark it as risky based on reputation data. The real answer isn’t in one tool—it’s in consistent, layered validation.

For example, an address like [email protected] might test as valid during SMTP handshake but should still be tagged as risky if it’s role-based and has a history of high bounce rates. That’s where systems like EmailListChecker’s bulk verification add value: they apply multiple signals, not just one test.

For deeper insight into domain reputation and email hygiene, refer to standards laid out by the SMTP RFC 5321 and reports from industry monitors like Spamhaus. These are the guardrails behind any trustworthy verification engine.

How Real-Time Verification Reveals True Address Status

Real-time SMTP checks simulate actual send attempts by connecting directly to the recipient’s mail server, exposing catch-all domains, greylisting, and temporary failures that static databases miss. This method reveals the true status of an email address—whether it’s truly invalid or just currently unreachable—reducing false positives by up to 30% compared to outdated data-only approaches.

Why Real-Time Checks Beat Static Databases

Static databases rely on cached records that can’t account for changes in server policies, temporary outages, or new catch-all configurations. You might see an address as "valid" in one system and "invalid" in another—especially when using Mailgun versus SendGrid—because each service interprets a transient failure differently. Real-time API verification resolves these mismatches by probing the actual server in real time.

Let’s say an address returns a "550 mailbox unavailable" error. A static database might mark it as invalid. But a real-time check notices the error is temporary (a common greylisting response) and flags it as "risky"—not dead. This prevents premature removal of potentially deliverable addresses.

Industry-standard tools like MxToolbox and Spamhaus validate email addresses using live SMTP handshakes, confirming that real-time checks reflect how email delivery actually works in practice.

How This Aligns With Email Deliverability Standards

Mailgun and SendGrid use different thresholds for marking an address as undeliverable. Mailgun may flag a temp error as a hard bounce; SendGrid might retry and delay the judgment. Without real-time verification, your list appears inconsistent across platforms.

By using a live verification API, you align your validation with actual SMTP behavior. This helps you catch issues like catch-all domains—where any email at a domain is accepted—which static tools can’t distinguish from real, active accounts.

The same process used by Mail-Tester during inbox placement checks is applied during real-time verification: connecting to the MX server, sending a full SMTP session, and reading the server’s response codes in real time. This gives you a clear, repeatable, and honest view of address status—no guesswork.

With real-time API verification, you eliminate mismatches between Mailgun and SendGrid, reduce bounce rates, and improve sender reputation—because only truly active, responsive addresses make it into your campaign lists.

Why DNS and SMTP Checks Alone Are Incomplete

You can’t fully trust email validation results from Mailgun or SendGrid if you only rely on DNS or SMTP checks. DNS-only validation misses temporary issues like greylisting or server-side throttling. SMTP-only validation might return "valid" for addresses behind catch-all domains—meaning the server accepts the email, but the inbox may never receive it. True accuracy requires real-time, multi-layer feedback beyond basic protocol checks.

DNS checks miss temporary delivery barriers

• DNS-only validation confirms an address format and domain existence, but it doesn’t see if the recipient server is temporarily blocking requests via greylisting.
• Greylisting delays delivery by temporarily rejecting emails and resuming only after a retry window—DNS checks can’t detect this behavior, leading to false positives.
• According to RFC 6950, greylisting is a standard practice, and many ESPs use it. Relying on DNS alone means you’re unaware of these transient failures.

SMTP checks can’t catch hidden invalid addresses

• SMTP-only checks connect to the mail server and confirm it accepts the address—but that doesn’t mean it’s a real, active inbox.
• Many domains use catch-all configurations, which accept all incoming mail without verifying the user exists. An SMTP check may pass, but the message still won’t reach its intended recipient.
• Mailgun and SendGrid may report these as valid, but they don’t account for whether the address actually belongs to a real person. This leads to wasted sends and poor deliverability.
• To confirm real inbox delivery, you need active monitoring—like checking if messages land in inboxes, not just servers.

That’s why combining DNS, SMTP, and real-time inbox placement testing is essential. A single-layer check, no matter how fast, gives incomplete results. You’re not just validating syntax or server responsiveness—you’re verifying whether an email can actually be delivered to a real user.

For teams using Mailgun or SendGrid, this means the best validation happens outside the platform. Tools like bulk verification and inbox placement testing simulate real-world send behavior and detect issues your ESP’s native checks might miss. They include SMTP, DNS, and active inbox checks—all in one.

Resolving Discrepancies: A Step-by-Step Process

Run your list through Emaillistchecker.io’s bulk verification tool to get a consistent, real-time assessment across all email types. Compare the results with Mailgun and SendGrid separately to spot conflicting verdicts. Investigate discrepancies by checking for catch-all domains or greylisting—common causes of inconsistent validation. Use inbox-placement testing to confirm if emails actually reach inboxes. Flag risky or catch-all addresses for removal or manual review.

Step-by-Step Verification Process

1. Run your list through Emaillistchecker.io’s bulk verification tool at https://emaillistchecker.io/bulk-verification. This gives you a single, unified baseline using real-time SMTP checks across 100+ domains. Unlike Mailgun or SendGrid, which may interpret bounces differently, Emaillistchecker.io evaluates validity, deliverability, and risk with consistent logic.
2. Compare outcomes with Mailgun and SendGrid results on the same list. Note where one service marks an address as valid, while another flags it as risky or invalid. These differences often stem from how each platform handles greylisting, catch-all detection, or temporary failures.
3. Identify conflicting addresses—for example, an email marked as valid in Mailgun but as "risky" in SendGrid. These are your priority cases. Check whether the domain returns a "catch-all" response (accepts all emails) by using tools like MxToolbox or RFC 5321, which standardizes SMTP behavior in email routing.
4. Investigate root causes. If a domain is catch-all, it often leads to false positives. Some platforms assume all addresses are valid; Emaillistchecker.io flags them as high-risk. If greylisting is in use, initial attempts fail, but deliveries may succeed later—this causes inconsistent results across services with different retry policies.
5. Use inbox-placement testing to validate delivery. Run your list through inbox-placement testing to see if messages land in real inboxes—not spam, not dropped. This step confirms whether a "valid" email actually gets delivered, bypassing sender reputation or filtering edge cases.
6. Flag and act. Remove catch-all addresses. Review risky emails for manual validation. Maintain a clean, deliverable list that reduces bounce rates, improves sender reputation, and avoids inbox filtering.

The Role of Catch-All Domains and Greylisting in Conflicts

When Mailgun accepts an email but SendGrid flags it as unreliable, one common culprit is a catch-all domain or greylisting. Catch-alls accept any address, making SMTP checks pass even for invalid ones. Greylisting temporarily rejects new senders, causing short-term testing to misfire. These two behaviors create mismatched validation outcomes across platforms.

Catch-All Domains Fool SMTP Checks

Some domains are configured to accept all incoming mail—these are catch-all domains. SMTP validation tools like Mailgun see a successful connection and consider the address valid. But that doesn’t mean the email actually reaches a real person. In fact, many of these addresses go to spam folders or bounce silently. A 2022 study by Return Path noted that catch-all domains contribute to a significant fraction of low engagement and high bounce rates in bulk campaigns.

This is why relying solely on SMTP verification is risky. A successful connection doesn't guarantee deliverability or inbox placement. You might score a hit on Mailgun’s validation, only to find your messages end up undelivered or marked as spam by SendGrid.

Greylisting Creates Temporary False Negatives

Greylisting is a common anti-spam measure where an MTA temporarily rejects a message on first contact, expecting the sender to retry after a delay. If your validation tool or test sends a message and doesn’t retry, the address appears invalid—even though it is real and functional.

Mailgun and SendGrid may handle retries differently. If Mailgun’s system retried automatically or tested over time, it might accept an address. SendGrid’s more aggressive short-term testing could flag the same address as unreliable during a retry window. This inconsistency is especially common during campaign dry runs or real-time API checks.

Greylisting is not a flaw—it’s an industry-standard spam defense. But it exposes weaknesses in validation logic that rely on single attempts. The solution? Use tools that test over time or simulate real sending behavior. EmailListChecker’s bulk verification includes retries and behavior modeling to catch these edge cases early.

Understanding both catch-all domains and greylisting helps explain why two platforms can give contradictory results. The answer isn’t to pick one tool over the other—it’s to verify with confidence, using a service that respects the full spectrum of email infrastructure quirks.

What Happens When You Ignore Verification Discrepancies?

You're risking your sender reputation, lower inbox placement, and misleading campaign metrics when you ignore why Mailgun and SendGrid report different results for the same email list. A 30% or higher bounce rate from invalid addresses can flag your domain to major ISPs. Inconsistent list quality may drop your inbox placement by 5% to 15%, and inflated open rates from undeliverable emails distort performance reporting, making it hard to identify real issues.

Higher Bounce Rates Trigger Reputation Penalties

If your list has 30% or more invalid addresses, you're operating well above the typical threshold that ISPs consider safe. Even a small spike in hard bounces can trigger automatic scrutiny from platforms like Gmail and Outlook. The underlying risk isn’t just wasted sends—it’s that ISPs use bounce patterns as a core signal when evaluating sender reputation. Once you’re labeled unreliable, recovery can take weeks or months.

SPF, DKIM, and DMARC alignment are only effective if the addresses you’re sending to are valid. Sending to known invalid or forged addresses undermines your domain authentication and makes it easier for ISPs to classify you as spam. For context, tools like MxToolbox or the Spamhaus Project help track reputation signals, but they rely on consistent send behavior. If your list quality varies between delivery providers, that inconsistency itself becomes a red flag.

Inaccurate Reporting Masks Real Delivery Problems

When one provider (like Mailgun) marks an email as valid and another (like SendGrid) flags it as undeliverable, you end up collecting open and click data from addresses that were never properly tested. This inflates your open rate, but it doesn’t reflect real engagement. Your reports show success, but your actual delivery is lower than you think.

As an industry-standard practice, most email service providers measure delivery success through both technical validation and behavioral signals. Relying on conflicting validation results means your data model becomes skewed—what looks like strong engagement might just be a few successful sends amid dozens of undelivered messages. The fix isn't more emails; it's smarter data.

Use a third-party verification tool to cross-validate results before sending. Bulk verification helps standardize list quality across senders like Mailgun and SendGrid. With 98.9% accuracy, Emaillistchecker.io catches invalid, role-based, and disposable addresses before they hurt your delivery. You can also test inbox placement with inbox-placement testing to see how your campaigns perform across major inboxes.

How Emaillistchecker.io Fixes the Conflict Problem

You’re getting conflicting validation results between Mailgun and SendGrid because each service uses different heuristics and timing windows—Mailgun may flag an address as invalid while SendGrid sees it as valid. Emaillistchecker.io resolves this by performing independent, real-time SMTP and DNS checks that don’t rely on either platform’s internal logic. Every address is verified with full evidence, and deliverability is confirmed via inbox-placement tests, so you know not just if an email exists, but if it actually lands in the inbox.

Clear, Evidence-Backed Verification

• Each email is validated via real-time SMTP and DNS checks—no guesswork or caching delays.
• Every address gets a definitive verdict: valid, invalid, catch-all, or risky—no ambiguity.
• Verdicts are backed by actual server responses: we don’t infer; we report what the mail server told us.
• Unlike Mailgun or SendGrid, which may use score-based systems with proprietary rules, ours is transparent and repeatable.

Deliverability Confirmed, Not Assumed

• After validation, inbox-placement testing confirms whether an email actually arrives in real inboxes, not just the server’s acceptance.
• Test results are based on actual delivery patterns across major providers—like Gmail, Outlook, and Yahoo—just like those tracked by Return Path in industry benchmarks.
• Use the inbox-placement tool to simulate real send behavior before your campaign launches.
• This stops the conflict: if an email passes validation but fails inbox placement, you’ve found a false positive—common when providers like SendGrid accept catch-all addresses as valid.

Want to align your Mailgun and SendGrid outcomes? Start with a bulk check: upload your list and see consistent results across your stack.

Best Practices for Consistent Email List Hygiene Across Platforms

You can resolve inconsistencies between Mailgun and SendGrid’s validation results by verifying your list with a third-party tool like Emaillistchecker.io before sending. This catches invalid, catch-all, and role-based addresses early, reduces bounces, and improves sender reputation across platforms. Run weekly comparisons between your ESPs and Emaillistchecker.io to spot discrepancies and adjust your filtering logic.

Apply a consistent, multi-layer verification workflow

• Run your email list through Emaillistchecker.io’s bulk verification before sending to identify invalid, risky, or catch-all addresses that could cause SendGrid and Mailgun to disagree.
• Compare validation outcomes weekly: if Mailgun flags an address as valid but SendGrid returns a hard bounce, verify it with Emaillistchecker.io to determine whether it’s a catch-all, role account, or temporary issue.
• Exclude catch-all domains and role accounts (e.g. admin@, support@, info@) unless your campaign specifically targets them—these accounts often accept mail but don’t engage, hurting deliverability and skewing engagement metrics.
• Re-validate your entire list every 60–90 days using Emaillistchecker.io’s real-time verification API to account for address changes, domain closures, and user churn.
• Use inbox placement testing via Emaillistchecker.io’s inbox placement reports to validate whether your messages are landing in inboxes—or spam folders—across major providers.

Align your filtering logic with industry standards

Discrepancies between Mailgun and SendGrid often stem from how each platform treats edge cases like MX record setups, greylisting, or temporary failures. According to RFC 5321, SMTP servers are not required to accept mail from non-existent addresses immediately—this means temporary failures can affect validation results.

Let’s be clear: no single ESP is perfect. Mailgun may accept a domain with a catch-all setup as “valid” due to a successful TCP handshake, while SendGrid may reject the same address during a later delivery attempt. Emaillistchecker.io uses multiple checks—SMTP, domain reputation, and syntax—to give you a more accurate signal than either ESP alone.

Use the Emaillistchecker.io integrations with Mailchimp, HubSpot, or Klaviyo to feed clean lists directly into your campaigns, reducing friction. You’ll see fewer bounces, less time spent on sender reputation recovery, and better inbox placement over time.

The Bottom Line: Consistency Comes from Independent Verification

Mailgun and SendGrid are both reliable in their own right, but their validation rules differ subtly. These differences can lead to inconsistent outcomes—even for the same email address.

Discrepancies aren’t errors. They’re symptoms of a system that relies too heavily on a single source. When one tool marks an address as valid and another flags it as risky, the real issue is the lack of a neutral, independent check.

Independent verification through a trusted third-party API removes the bias inherent in relying on a single platform. It ensures consistency across your lists and directly improves inbox placement over time.

Keep reading

• Email verification integrations for ESPs, CRMs and marketing tools (complete guide)
• AI-Powered Header Detection for Email List Validation in Excel Files
• How to Integrate Signed URLs with Email Verification Result Storage
• Integrate Email Verification in Step Functions for Automated Campaigns
• Email Verification Platform with Sunset Flow Integration

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

Frequently asked questions

Why does Mailgun say an email is valid but SendGrid says it’s risky?

Mailgun may accept it based on SMTP response, but SendGrid detects it as a role account or catch-all domain, which it flags as risky.

Can catch-all domains pass validation in both Mailgun and SendGrid?

Yes — both can mark them as valid due to accepted SMTP responses, but this is a known risk for deliverability.

How does real-time verification reduce false positives?

It simulates actual sending and detects greylisting, temporary failures, and catch-all domains in real time.

Do inbox-placement tests improve deliverability?

Yes — they validate whether messages land in inboxes, not just whether addresses are format-valid.

Can I integrate Emaillistchecker.io with Mailgun and SendGrid?

Yes — it has native integrations with both platforms, plus Mailchimp, HubSpot, and Klaviyo.

What’s the accuracy of Emaillistchecker.io?

It maintains 98.9% accuracy through real-time verification and multiple validation layers.

Do unused verification credits expire?

No — purchased credits never expire, allowing flexible scheduling of list hygiene checks.

Is there a free way to test Emaillistchecker.io?

Yes — you get 100 free verifications to start, no credit card required.

How often should I clean my email list?

Every 60 to 90 days, or before major campaigns, to maintain high deliverability and sender reputation.

What’s the difference between a role account and a disposable email?

Role accounts (e.g. info@) are valid but high-risk; disposable emails (e.g. mailinator.com) are temporary and often used for spam.

Why does SendGrid flag role accounts as risky?

They often have high bounce rates and low engagement, damaging sender reputation if used broadly.

Can Emaillistchecker.io detect disposable domains?

Yes — it identifies disposable and temporary email domains, helping prevent spam traps and invalid addresses.