Why Are Mailgun and SendGrid DSN Bounce Reports So Different?

You're automating email list hygiene, and your system pulls in DSN bounce reports from both Mailgun and SendGrid. But the data doesn’t line up. One says “550 5.1.1” — the other says “invalid recipient” with a code you’ve never seen. Why does the same failed delivery look so different?

Both platforms use DSNs — the standard for reporting email delivery outcomes — but they interpret and structure the fields differently. What’s a clean, actionable status in one system might be buried in a diagnostic-code in the other. This isn’t just inconsistency. It’s a friction point for any process that needs to normalize bounce data across providers.

You’re not just comparing syntax. You’re trying to build a single source of truth for list hygiene. Without mapping these fields correctly, you risk misclassifying bounces, missing real invalid addresses, or over-cleaning active ones.

Key takeaways

  • Mailgun and SendGrid use the same DSN standard, but apply different field mappings and semantic interpretations, making direct comparison impossible without normalization.
  • Field names like status, reason, and diagnostic-code carry different meanings between the two — for example, SendGrid often includes human-readable reasons, while Mailgun relies more on SMTP status codes.
  • Automated list hygiene fails without explicit mapping rules for each DSN field, especially when handling non-delivery reports at scale.

What Does a DSN Bounce Report Actually Tell You?

A DSN bounce report is a standardized, machine-readable response from a receiving mail server when an email fails to deliver. It includes the original recipient, the SMTP response code, the delivery status (bounce or delay), and the reason for the failure—helping you diagnose whether an address is invalid, temporarily unavailable, or blocked. You can use this data to clean your list and improve deliverability.

How DSN Reports Break Down Delivery Failures

When an email doesn’t reach its destination, the receiving server sends a DSN (Delivery Status Notification) back to the sender. This report isn’t just a "failed" flag—it gives you structured details, like the exact SMTP error code (e.g., 550 means the mailbox doesn’t exist), the original recipient, and whether the failure was permanent (hard bounce) or temporary (soft bounce).

Hard bounces indicate permanent issues—like a non-existent email address, a domain that doesn’t exist, or a rejected mailbox. Soft bounces point to temporary problems: a full inbox, a server timeout, or a message too large. These can often be retried, but repeated soft bounces degrade sender reputation over time.

Why Understanding the Fields Matters

Different ESPs (like Mailgun and SendGrid) structure their DSN reports slightly differently. Mailgun’s reports include fields like status, action, remote_mta_ip, and failed_recipient. SendGrid’s reporting uses similar but not identical field names—like status, type, and reason. They both follow the same underlying IETF standards (RFC 3463 and RFC 3464), meaning the core mechanics are consistent, but how you parse and act on the data depends on your system’s setup.

For example, a Mailgun bounce with status=550 and action=failed means "recipient unknown." A SendGrid status=551 would mean "user not local." While the codes differ, both signal a hard bounce. If you’re building an automation pipeline, this means you need to normalize these codes across platforms to avoid false alerts or overlooked bounces.

Understanding these fields helps you distinguish between a temporary blip and a cleanable list error. You can filter out invalid addresses before they hurt your sender reputation. For deeper insight, tools that analyze bounce patterns—like email providers that track trends across thousands of deliveries—can help you spot broader deliverability risks.

For teams managing bulk sends, validating your list in advance cuts down on these failures. Using real-time validation or bulk list cleaning can prevent many bounces before they happen. Explore a high-accuracy email validation tool to keep your data clean: clean your list with precision.

Mailgun DSN Bounce Report Fields: What Each One Means

You’ll find five key fields in a Mailgun DSN bounce report: mailgun-receiver (the failing address), mailgun-delivery-status (failed/delivered/delayed), mailgun-delivery-reason (why it failed), mailgun-delivery-code (the SMTP code, like 550), and mailgun-message-id (a unique tracker for your message). Together, they let you diagnose delivery issues at scale and improve sender reputation.

Decoding the DSN Fields in Practice

  1. Check the receiver with mailgun-receiver. This is the exact email address that bounced. Use it to confirm if the address was mistyped or belongs to a high-risk domain. Cross-reference it with your list to remove invalid entries before the next send.
  2. Assess the delivery status via mailgun-delivery-status. A value of failed means the message didn’t reach the destination. delayed indicates temporary issues—common with greylisting or rate limits. delivered means success, but still verify inbox placement.
  3. Read the reason in mailgun-delivery-reason. Words like user unknown, mailbox full, or blocked reveal the type of failure. These help you distinguish between temporary issues and permanent address problems. For example, a user unknown is a clear signal to remove the address.
  4. Look up the SMTP code in mailgun-delivery-code. Codes like 550 (user unknown), 450 (try again later), or 552 (quota exceeded) are defined in SMTP standards. Understanding these codes lets you automate filtering rules—like flagging 550s as permanent failures.
  5. Track the message using mailgun-message-id. This ID links back to the original send request in Mailgun’s logs. It’s essential for troubleshooting when the message failed after routing or was rejected by a filtering system.

Matching Fields Across SendGrid

While SendGrid’s DSN fields are similar, their naming differs: recipient instead of mailgun-receiver, status instead of delivery-status, and smtpResponse for the code. The core logic remains the same—your system should map these fields consistently when processing bounce reports at scale. See how RFC 3463 defines DSNs for the full context on standardized delivery status codes.

For a full picture of email deliverability, pair these insights with real-time verification. Use real-time email validation to catch issues before sending, and bulk list cleaning to remove risky or invalid addresses in advance. This reduces bounces, improves sender reputation, and keeps your messages out of spam folders.

SendGrid DSN Bounce Report Fields: Decoding Each Value

SendGrid’s DSN bounce report fields tell you exactly why an email failed to deliver. The key fields are delivery (success or failure), status (SMTP code), reason (human-readable explanation), recipient (email address), and smtp-id (message correlation ID). These values help you trace issues, clean lists, and improve sender reputation. For deeper list hygiene, use real-time verification to catch invalid addresses before sending.

The Step-by-Step Process of Interpreting Bounce Data

  1. Check the delivery status — If it says failed, the email did not reach the recipient's server. A delivered status means the server accepted the message, but that doesn’t guarantee inbox placement. This is your first filter: only act on failures.
  2. Examine the SMTP status code — It’s a numerical response from the receiving mail server, such as 550 (mailbox not found) or 451 (temporary error). These codes are standardized in RFC 5321, the core SMTP specification. Knowing the code helps you categorize the reason: permanent, temporary, or policy-based.
  3. Read the reason field — This is the plain-language explanation from the server. Examples: “User unknown”, “Message rejected”, or “Rejected due to policy”. It’s useful for quick triage but can vary in detail. Combine it with the status code for accuracy.
  4. Identify the failing recipient — The recipient field shows the exact email address that bounced. This is critical for updating your list. If a user changes their email, you’ll need to know which one failed to avoid future friction.
  5. Use the smtp-id to trace the message — This ID links the bounce back to the original email, including timing and routing details. It’s essential for debugging deliverability issues, especially when multiple bounces overlap. Some email platforms use it for event correlation across logs.

Why This Matters for List Hygiene

Understanding these fields lets you differentiate between temporary issues (like a full inbox, 4xx codes) and permanent failures (5xx codes or “user unknown”). Acting only on permanent bounces improves your sender reputation and reduces the risk of being flagged by ISPs. Tools like bulk email list cleaning can pre-filter these errors before you send, saving time and boosting deliverability.

Key Differences Between Mailgun and SendGrid DSN Field Structures

You need to normalize DSN data from Mailgun and SendGrid because their field names and structures differ significantly. Mailgun uses mailgun-delivery-status and mailgun-delivery-reason, while SendGrid uses status and reason. Mailgun includes mailgun-message-id; SendGrid uses smtp-id for the same purpose. Both include recipient and SMTP status codes, but their naming and format make cross-platform parsing non-trivial. These discrepancies mean you can't treat DSNs from both services the same way without custom logic.

Field Naming and Prefixing

Mailgun prefixes all its DSN fields with mailgun-, a pattern that makes them distinct and easier to isolate. SendGrid, by contrast, uses bare field names like status, reason, and smtp-id. This difference isn’t just stylistic—it forces you to include logic that checks for prefixes when processing incoming DSNs. If you’re aggregating DSNs from multiple platforms, this prefixing adds a layer of normalization that must be handled programmatically.

For instance, when you receive a bounce, you might see mailgun-delivery-status: rejected in a Mailgun report, but status: blocked in a SendGrid one. The values may map semantically—rejected and blocked both mean delivery failed—but the field names differ. This means standard parsers won’t work across both platforms without adjustments.

Message ID and Recipient Handling

Both services include a message identifier, but they differ in name and format. Mailgun’s mailgun-message-id is a unique identifier tied to the original message; SendGrid’s equivalent is smtp-id. These IDs are critical for tracing delivery issues in logs, but their inconsistent naming complicates correlation across systems.

Recipients are also reported with similar intent but varying formats. Both include the recipient email address, but the exact field name and structure depend on the provider. SendGrid includes status codes in a consistent format (e.g., 5xx for permanent failures), while Mailgun uses a mix of codes and descriptive terms, requiring additional mapping.

These differences are documented in the IETF’s RFC 3463 and RFC 6522, which define the DSN standard but leave room for implementation variation. RFC 3463 lays the foundation, but actual field names and semantics are left to the service provider. That’s why you can’t assume interoperability.

Automating cleanup and monitoring of bounced emails is much easier when your system knows how to map fields across providers. Tools like bulk email list cleaning help by identifying invalid addresses early, reducing bounce rates before they even reach your ESP.

How to Normalize DSN Bounce Data Across Mailgun and SendGrid

You can align bounce data from Mailgun and SendGrid by extracting the recipient, SMTP status code (5xx for hard, 4xx for soft), and reason codes from both systems, then mapping standard SMTP responses like 550 to “user unknown” and 450 to “mailbox full.” Group all 5xx codes into a single “hard bounce” category, apply consistent field names like status_code, delivery_reason, and message_id, and use a central logic layer to standardize input before analysis.

Step-by-step: Normalize Bounce Reports

  1. Extract core fields from both platforms—Mailgun and SendGrid both include recipient, smtp_status, and reason in their DSN reports. You don’t need to parse raw SMTP logs; use the structured fields provided through webhook delivery.
  2. Map SMTP status codes to standard delivery outcomes—Use well-documented SMTP semantics: 550 means user unknown, 551 is user not found, 552 is mailbox full, and 450 indicates mailbox unavailable. RFC 5321 and RFC 5322 define these codes explicitly. This ensures consistency even when different providers label the same issue differently.
  3. Classify bounces by severity—Treat all 5xx codes as hard bounces. Do not split them at first. A single “hard bounce” category simplifies monitoring and list hygiene. Soft bounces (4xx) rarely need individual handling unless you’re tracking long-term delivery health.
  4. Standardize field names across sources—Rename all incoming fields to a consistent schema: recipients (plural), status_code (e.g., 550), delivery_reason (e.g., “user unknown”), message_id, and delivery_status (e.g., “failed” or “rejected”). This makes downstream processing uniform.
  5. Validate with trusted sources—Use the RFC 5321 and RFC 5322 as reference for proper SMTP semantics. These are the authoritative definitions—not marketing docs or tool guides.
  6. Automate normalization with a lightweight parser—Write a small function or use a tool like bulk email list cleaning to process incoming DSNs. These systems often handle bounce logic, but if you’re integrating manually, treat this as part of your delivery pipeline.

Why consistency matters

Unnormalized bounce data leads to false positives in list hygiene. One platform might report “user unknown” for a 550; another reports “invalid address.” Without normalization, you can’t reliably flag bad addresses or evaluate sender reputation. Standardizing ensures your data reflects real delivery issues, not format differences.

“The key to effective deliverability management is not just tracking bounces—but understanding what they mean across systems.” — Return Path (now Validity)

Why Raw DSN Data Isn’t Enough for List Hygiene

You can’t clean your email list effectively by relying only on DSN bounce reports—they only tell you what failed after delivery, often too late to protect your sender reputation. By then, spam traps, invalid addresses, or disposable domains have already eaten into your deliverability. Bounce data is reactive, not preventive. The real fix starts before you send.

DSN Reports Show Failure, Not Prevention

DSN reports from Mailgun and SendGrid record delivery failures after the SMTP handshake has already happened. That means your message was delivered to an inbox or rejected by the recipient’s server—either way, the damage may already be done. A single hard bounce from a high-volume list can trigger reputation flags at major ISPs, especially if it happens repeatedly.

Even when a bounce report marks an address as “undeliverable,” it doesn’t tell you why. Was it a typo? A forgotten role account? A catch-all mailbox that accepts any email? Some systems return the same “550” code for any rejection, but the underlying cause varies widely. Without root-cause insight, you’re left guessing.

Proactive Verification Beats Reactive Cleanup

Let’s be clear: you can’t proactively filter out disposable emails, catch-all domains, or role accounts using DSN data alone. These are latent risks that only surface after you’ve already sent. That’s why bulk verification before sending is more effective than relying on post-send bounce reports.

Using a tool that checks addresses in real time—like bulk email list cleaning—lets you flag and remove invalid or risky addresses before they ever hit your sender pool. This includes detecting temporary aliases, outdated mailboxes, and domains known for disposable or role-based email creation, which are common in list data pulled from lead magnets or third-party sources.

For example, RFC 6542 defines how bounces should be reported, but it doesn’t require senders to prevent delivery to known bad addresses. You’re on the hook for ensuring list quality, not just tracking failures. This is where a real-time verification API like ours adds value—it validates at the point of entry, reducing bounce rates and improving inbox placement from day one.

Think of DSN reports as a logbook of events after the fact. They’re not your primary defense. Your real firewall is verifying every address before you send. That’s how you avoid hitting deliverability walls in the first place.

Use Email List Validation to Replace DSN Cleanup

You don’t need to wait for DSN bounce reports to clean your list. Email List Validation checks every address in real time or via bulk verification, catching invalid, catch-all, disposable, and role-based emails before you send. This prevents bounces at the source and protects your sender reputation—no late-stage cleanup required. Unlike DSN reports that react after delivery, this approach stops issues before they happen.

Why DSN Reports Are Too Late

DSN bounce reports tell you what went wrong after the email was sent—but they don’t help you avoid the failure in the first place. By then, your sender reputation has already taken a hit if you’re sending to invalid or role-based addresses. Bounces from roles like admin@, sales@, or info@ are common and often overlooked in DSN logs, but they hurt deliverability over time.

According to RFC 3463, bounce codes like 5.1.1 (unknown user) or 5.7.1 (rejected) are standard in DSNs—but seeing them isn’t the point. The point is knowing they *won’t appear at all* if you verify addresses before sending.

How Real-Time Verification Stops Bounces Before They Start

Let’s say you're planning a campaign. Instead of trusting a list from a third-party tool or a sign-up form, you can run it through Email List Validation. It uses SMTP checks to confirm domain existence and inbox responsiveness, plus pattern analysis to flag disposable domains or role accounts. A valid result means the address likely accepts messages. A catch-all result means the domain accepts all emails—even typos—so you’ll never get a bounce, but delivery rates will still be poor.

You can run this at scale with the bulk email list cleaning tool, or integrate the real-time verification API directly into your signup or CRM flow. Every address is checked in seconds, and you get clear verdicts: valid, invalid, catch-all, disposable, or risky.

This isn’t just about reducing bounce rates. It’s about controlling your sender reputation from day one. ISPs and email providers track sender behavior over time. Sending to a list with 12% invalid addresses signals poor list hygiene, regardless of content quality. With Email List Validation, you reduce sender risk before you ever hit send.

And yes, this works with tools like Mailgun and SendGrid. You may still receive DSN reports—but they’ll be far cleaner, with fewer false positives and fewer deliverability red flags. That’s not a feature. That’s a consequence of doing email hygiene right.

Email List Validation vs DSN Bounce Reporting: A Real-World Comparison

You don’t need to wait for bounces to fix bad emails. While Mailgun and SendGrid DSN reports tell you what failed after delivery, Email List Validation catches invalid addresses before you send—reducing bounce rates by up to 98.9% in a single verification session. It’s not a fix; it’s a prevention.

Why DSN Reports Fall Short

  • DSN reports are reactive: they only arrive after a message fails to deliver, often days later, making them too late to prevent campaign damage.
  • Mailgun and SendGrid DSN fields (like status, diag, reason) vary in format and meaning, requiring complex normalization logic across platforms.
  • Even when normalized, DSN data is incomplete—catch-all domains, temporary failures (like greylisting), and role accounts often go unclassified, leaving you guessing.
  • Many bounces are soft or transient—some users never recover their inbox. You’re left cleaning up a mess that could’ve been avoided.

The Proactive Alternative: Email List Validation

  • Verifications happen before sending—no delays, no guesswork. You can clean a list of 10,000 emails in under 5 minutes with bulk email list cleaning.
  • Results are standardized: each address gets a clear verdict—valid, invalid, risky, or catch-all—regardless of which email platform sent it.
  • 98.9% of invalid addresses are caught in one session, according to our internal benchmarks and consistent with industry-standard deliverability practices (see RFC 6522 on bounce processing).
  • Unlike DSN reports, this system detects disposable domains, outdated roles (e.g., admin@), and syntax issues that never make it past SMTP.
  • Real-time API integration allows you to validate on signup, in real time—preventing list decay at the source.

Let's be clear: you’re better off not sending to unverifiable addresses than chasing bounces later. While DSN data gives a post-mortem, Email List Validation gives you the tools to avoid the autopsy.

Integrating Email List Validation with Mailgun and SendGrid

You can use Email List Validation to clean your list before sending via Mailgun or SendGrid, reducing bounces and protecting your sender reputation. The process starts with bulk verification to remove invalid, risky, or disposable emails. Then, pull the clean list into your ESP using native integrations or APIs. This ensures only deliverable addresses are used, directly lowering hard bounces and improving inbox placement. SMTP standards define how bounce reports are structured, but their formats differ between providers—understanding these distinctions helps you parse feedback correctly.

Step-by-step: Prepare your list with Email List Validation

  1. Verify your list before importing into Mailgun or SendGrid. Use the bulk verification tool to scan thousands of emails at once. This catches invalid addresses, catch-all domains, role accounts, and disposable domains before they hurt your reputation.
  2. Review the results. The tool reports each email as valid, invalid, catch-all, or risky. Valid emails are likely to receive your message; invalid ones will bounce. Catch-alls appear safe but may absorb messages without delivery, lowering deliverability. Risky addresses, such as those with typo-squatting patterns, are filtered out.
  3. Download and import clean data. Export only verified, valid emails. Import this list into Mailgun or SendGrid via API, CSV, or direct integration. This reduces hard bounces by removing dead addresses early, which keeps your sender score healthy.
  4. Automate ongoing list hygiene. Connect Email List Validation to Mailchimp, HubSpot, Klaviyo, or SendGrid through the integration hub. New signups are auto-verified, and list decay is managed without manual effort.

Why this matters for DSN bounce reports

Mailgun and SendGrid both send DSN (Delivery Status Notification) bounce reports. But their fields differ. You’ll see different codes (e.g., 550 vs 5.1.1) and reporting structures. Understanding these variations lets you map feedback accurately. For example, a 5.1.1 error is a routing issue; 550 means the mailbox doesn’t exist. If you don’t clean your list upfront, you’ll see inflated bounce counts from false negatives—especially with catch-alls or role accounts that appear online but aren’t real recipients.

Maintain a Clean List Before Sending — Don’t Wait for Bounces

DSN bounce reports help track delivery failures, but they only reveal issues after the damage is done. By then, your sender reputation may already be at risk.

The real win comes before sending. Cleaning your list proactively stops invalid addresses from ever entering the pipeline. This reduces bounce rates, protects deliverability, and improves inbox placement.

Email List Validation checks your list for validity, catch-alls, role accounts, and disposable domains — all before you send. It identifies issues with 98.9% accuracy, helping you maintain a high-quality, trustworthy email list.

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Frequently asked questions

What is a DSN bounce report?

A DSN (Delivery Status Notification) is a standardized email message sent by a receiving server when an email fails to deliver. It contains status codes, reasons, and recipient data to help diagnose delivery issues.

Why do Mailgun and SendGrid DSN fields differ?

Mailgun and SendGrid use different internal formats for DSNs. Their field names and structures vary, requiring manual or automated normalization for consistent analysis.

Can I use DSN reports to clean my email list?

Yes, but only reactively. DSN data shows failures after delivery attempts. Using tools like Email List Validation before sending is more effective and prevents reputation damage.

What is the best way to handle 5xx SMTP status codes in DSN reports?

Treat 5xx codes as hard bounces—indicators of permanently invalid addresses. Remove these recipients from your list immediately to maintain sender health.

How accurate is Email List Validation?

Email List Validation achieves 98.9% accuracy in identifying valid, invalid, catch-all, and risky addresses through real-time and bulk verification.

Do I need to pay for an Email List Validation subscription?

No. You get 100 free verifications to start, and purchased credits never expire. This allows you to verify lists on demand without urgency.

Can Email List Validation integrate with SendGrid?

Yes. Email List Validation integrates with SendGrid, allowing you to verify lists before sending and reduce bounce rates across your campaigns.

What’s the difference between a catch-all and a role address?

A catch-all address accepts all emails sent to an domain, even invalid users. A role address (e.g. sales@) is a shared mailbox for a function, not a person. Both can inflate bounce rates if not filtered.

How often should I verify my email list?

Verify your list before major campaigns, and periodically (e.g. every 3–6 months) to maintain hygiene, especially as users change email addresses or roles.

Is there a way to automate list verification across platforms?

Yes. Email List Validation offers integrations with Mailchimp, HubSpot, Klaviyo, and SendGrid, enabling automated verification workflows without manual export/import.