Why does Mailgun reject emails with 451 4.7.0 while SendGrid delivers successfully?

You’ve sent the same email to the same list—identical content, same headers, identical timing—yet Mailgun returns a 451 4.7.0 error while SendGrid delivers without a hitch.

This isn’t a problem with your message. It’s not even a typo. It’s a server-side throttling signal: a temporary rejection based on how quickly you’ve been sending, not what you’re sending.

You’re not alone. Many teams see this gap in behavior between Mailgun and SendGrid—even when everything else is the same. The difference lies in how each platform handles sending volume, reputation, and rate limits over time.

Key takeaways

  • The 451 4.7.0 error means temporary delivery delay, not failure—triggered by rate limits, not content
  • Mailgun enforces stricter sending thresholds per domain and IP than SendGrid
  • Even identical emails can be rejected by one service and accepted by another based on historical sending behavior

How throttling works: What 451 4.7.0 really means

SMTP error 451 4.7.0 means the receiving server temporarily refused your email due to rate limits or sender policy — not because the address is invalid. It’s a soft rejection, not a bounce. The server expects you to retry later, often after a delay of minutes to hours. This is normal behavior during high-volume sends, especially from platforms like Mailgun.

Why throttling happens — and why it’s not a failure

You’re seeing 451 4.7.0 because the receiving server, like Gmail or Outlook, is limiting how many emails it accepts from a single sender in a short time. This is a defensive move against spam floods and bulk attacks. It doesn’t mean your domain is blocked, or your message is bad — just that you sent too much, too fast.

These limits aren’t fixed. They’re dynamic and based on real-time behavior, sender reputation, and historical patterns. A server might let you send 100 emails per minute at 3 a.m., but throttle you at 10 per minute during peak hours. You can’t predict exact limits, and they’re rarely published. That’s why some senders (like SendGrid) succeed while others (like Mailgun) hit 451 4.7.0 — it depends on how each platform manages sending pace and reputation.

How to avoid throttling, even when you're sending at scale

Throttling is a feature, not a bug. The goal is to keep inboxes clean. But if it slows your campaign, it’s time to reassess your sending strategy. One common fix is to throttle your own sends — reduce the rate to stay under the radar.

You can also check if your sender IP or domain has reputation issues. Tools like MxToolbox help you spot blacklisting or SPF/DKIM misconfigurations. But even clean setups get throttled if volume spikes. That’s where list hygiene matters.

Before sending to thousands, verify every email address in bulk. You’ll catch inactive, malformed, or role-based addresses that can hurt your reputation. With a tool like bulk email list cleaning, you remove risk before sending — helping you avoid triggering rate limits in the first place. You’re not just reducing bounces; you’re improving deliverability by sending only to valid, active inboxes.

Why Mailgun's throttling is more aggressive than SendGrid's

Mailgun enforces stricter rate limits on new domains and IPs, especially during warm-up, while SendGrid's infrastructure handles higher volumes with automated reputation scaling. This means sudden spikes in sending from a new Mailgun account often trigger a 451 4.7.0 error—throttling—where SendGrid may still deliver. Mailgun evaluates sending behavior more rigidly per IP and domain, making consistency critical to avoid blocks.

Mailgun's new-domain rate limits are harder to pass

When you start using Mailgun, especially with a fresh domain or IP, the system applies tighter controls. New senders are expected to ramp up slowly—sending only a few hundred messages per day at first. A sudden increase, even if within your plan’s total limit, can still trigger throttling because Mailgun treats it as inconsistent or high-risk behavior during the warm-up phase.

This is not unique to Mailgun—you can see similar practices in industry guidelines on sender reputation (e.g., RFC 6655, which defines best practices for SMTP delivery and rate adherence). New senders are expected to avoid overwhelming receiving servers during initial deployment.

SendGrid’s infrastructure better handles burst traffic

SendGrid’s backend is built for larger-scale, high-throughput use. It uses dynamic sender reputation models that adjust in real time based on feedback from mailbox providers. This means it can absorb temporary traffic spikes better—even if your sending pattern varies or hits peaks—without immediately throttling.

Mailgun’s stricter per-IP and per-domain evaluation means it’s less forgiving of irregular patterns. If you send 10,000 emails in 10 minutes from a new Mailgun IP, you’ll likely hit a 451 4.7.0 error. SendGrid might allow it during a warm-up window, depending on your historical performance signals.

For better delivery hygiene, validate your email list before sending. A clean list with fewer invalid or risky addresses reduces the chance of spikes and improves sender reputation. Try bulk email list cleaning to catch problematic addresses before they impact your sending performance.

The real root cause: Sending too fast with a weak sender reputation

You’re seeing Mailgun return 451 4.7.0 throttling while SendGrid delivers because Mailgun’s reputation engine detects early signs of poor sending behavior—like sudden volume spikes from a new or unproven IP—more quickly than SendGrid does. Even if your domain is clean, sending too fast without a track record of engagement or bounce history triggers delivery limits.

Sender reputation starts before your first email

Most inbox providers evaluate reliability not just by your domain name, but by how long your IP has been active, how many people open or mark your messages as spam, and whether your volume grows too fast. If you’ve just started sending or haven’t built a consistent sending pattern, a sudden surge—even to just a few thousand emails—can look suspicious.

Mailgun’s systems are designed to react faster to deviations from typical behavior than some competitors. The same email that hits a SendGrid threshold after 100k sends might get throttled by Mailgun after 10k if the IP has little to no historical engagement. This isn’t a flaw—it’s how reputation-based filtering works.

Volume alone isn't the problem; timing and signal matter

Sending 50,000 emails in one hour is safe if you've sent 500,000 before with strong open rates. The same volume from an IP with no past history? That triggers throttle policies. This is why inbox providers use time-based thresholds and reputation scores—what seems normal to a sender can be a red flag to a system evaluating abuse patterns.

Even well-known tools like SendGrid have thresholds that allow some burst sending from new senders. But Mailgun’s architecture prioritizes early anomaly detection, often acting before the sender realizes there’s an issue. Their throttling is usually a sign of good hygiene, not poor setup.

It’s not about choosing one platform over another. It’s about understanding how each evaluates reputation. You can reduce throttling risk by warming up IPs gradually, monitoring engagement, and verifying your list before sending.

Use a bulk email list cleaning service to filter out invalid, catch-all, and low-engagement addresses before sending. A clean list improves your sender reputation and prevents throttling. It’s the most effective step to take—before any send—even if you’re using a trusted provider like Mailgun or SendGrid. Real-time verification also helps you catch risky addresses on the fly.

For more on how reputation affects deliverability, refer to the RFC 6409 standard on email authentication and delivery policy. It details how systems evaluate sender trustworthiness over time.

How to fix 451 4.7.0 throttling on Mailgun

Mailgun returns 451 4.7.0 throttling when your sending rate exceeds its limits or your sender reputation is weak. To resolve this, reduce your email volume to under 300 messages per minute, warm up new IPs gradually, and monitor real-time engagement to avoid reputation damage. You’ll restore deliverability across all platforms.

Step-by-step fixes for 451 4.7.0 throttling

  • Check your sending rate against Mailgun’s documented limits: new domains should stay under 100 emails per minute initially, scaling up to 300 as reputation builds. Sending faster risks triggering throttling by default.
  • Warm up new domains and IPs with consistent, low-volume email bursts over 5–10 days. Start with 10–20 emails per day, increasing daily by 20–30% until reaching your target volume. This signal tells receiving servers you’re legitimate.
  • Use Mailgun’s built-in delivery reports and third-party tools like Spamhaus or MXToolbox to monitor blacklists and feedback loops. A high bounce or complaint rate degrades sender reputation, even if your volume is within limits.
  • Verify your email list quality before sending. Invalid addresses, catch-all domains, and disposable emails inflate bounces and hurt performance. Clean your list with bulk verification to remove risky addresses before delivery.
  • Ensure your DNS records (SPF, DKIM, DMARC) are correctly configured. Misconfigurations cause authentication failures, making your emails vulnerable to throttling, even at low volume.
  • Monitor unsubscribe and engagement rates in real time. A sudden drop signals inbox placement issues. Low engagement over multiple sends can trigger throttling by Mailgun and rival providers alike.
  • If throttling persists, check whether your sending domain has shared IP history with high-volume or spam-suspected senders. Mailgun uses shared infrastructure—your reputation can be affected by others.

Why SendGrid delivers while Mailgun throttles

SendGrid and Mailgun use different reputation systems. SendGrid’s higher-tier plans include dedicated IPs and faster warm-up access, so a given sender may see higher thresholds. But both rely on sender reputation and sending behavior. If your Mailgun sending habits — volume, engagement, list quality — don’t match SendGrid’s best practices, the discrepancy will remain.

Use email list validation to prevent throttling before it starts

You’re getting a 451 4.7.0 throttling error in Mailgun while SendGrid delivers fine because your list contains invalid, role-based, or disposable email addresses. These degrade sender reputation and trigger rate limits. Cleaning your list before sending stops throttling before it begins. You don’t need to guess — automated validation catches the problem early.

Why bad addresses lead to throttling

Every email sent to an invalid, role, or disposable address increases your bounce rate. High bounce rates signal poor list hygiene to email providers. Even a few bad sends can push your sender reputation into the red, triggering throttling — especially with platforms like Mailgun that enforce stricter rate limits than others.

When a domain is configured with a catch-all response, Mailgun may return 451 4.7.0 not because the message is forged, but because the server is intentionally rate-limiting to prevent abuse. If your list has many such domains, you’re likely hitting those thresholds. The fix isn’t just in sending slower — it’s in sending cleaner.

How to catch bad emails before they cause trouble

Let’s be clear: throttling isn’t about the tech — it’s about quality. A list with just 5% invalid addresses can hurt your deliverability, especially at scale. Email List Validation scans your entire list in seconds, identifying invalid, catch-all, and role-based emails before they ever hit a server.

Our system uses real-time SMTP checks and domain intelligence to achieve 98.9% accuracy in identifying invalid and risky addresses. It checks against known disposable domains, role accounts (like admin@, info@), and domains that absorb all incoming mail without delivering it. This means fewer bounces, better sender reputation, and fewer throttling alerts.

It’s not just about fixing problems after they happen. If you clean your list before sending, you stop throttling before it starts. Tools like Mailgun’s own rate limits and SendGrid’s delivery metrics are designed to catch abuse — but they can’t tell the difference between a real user and a disposable email. A clean list does.

For teams that send at scale, this isn’t a luxury. It’s a baseline requirement. You can test inbox placement with our inbox-placement tool to see how clean your list is in real inboxes. Or, integrate Email List Validation’s real-time API into your signup flow to prevent bad emails from ever entering your system.

Check the results: clean your entire list in minutes and stop throttling before it begins.

Real-time email verification API: Prevent throttling before send

You’re seeing 451 4.7.0 throttling in Mailgun but SendGrid delivers because your list contains invalid or problematic addresses that trigger rate limits. By validating each email in real time before sending, you catch these issues early—especially catch-all addresses that appear valid but cause soft bounces when messages arrive. This stops bounce spikes before they grow, preventing throttling and preserving sender reputation.

How real-time validation stops 451 4.7.0 before it starts

Mailgun enforces strict policies around bounce volume. When too many messages are sent to addresses that either don’t exist or are silently dropped (like catch-alls), Mailgun’s systems flag the sender as a potential spam source. Once that happens, you get 451 4.7.0 responses—temporary delivery failure due to throttling. SendGrid may deliver successfully because it either tolerates higher bounce rates or has different rate-limiting thresholds, but that doesn’t mean the list is healthy.

Here’s the fix: integrate a real-time verification API into your sending workflow. Every time a new email is added or a campaign is sent, check it against live DNS and SMTP servers. This confirms the address is not only syntactically valid but also capable of receiving mail. Tools like Email List Validation’s API perform this check in under 100 milliseconds with 98.9% accuracy.

It’s not just about preventing hard bounces. Catch-all addresses—commonly accepted during syntax checks—can silently reject messages once delivered. If your campaign sends to hundreds of these, Mailgun will see a high rate of soft bounces (which are still counts toward threshold metrics). The result? Temporary delivery suspension and the 451 4.7.0 error, even if the address appeared valid earlier.

Why the difference between Mailgun and SendGrid isn’t a fix—it’s a symptom

SendGrid’s ability to deliver while Mailgun throttles isn’t a sign that SendGrid is better. It’s evidence that your list contains addresses that don’t actually receive mail. This is an inbox placement issue, not a transport problem. The real risk isn’t the error—it’s the damage to sender reputation if you keep sending to those addresses.

Use real-time validation to identify and remove problematic emails before they leave your system. This improves deliverability across all providers, not just Mailgun. It’s not about optimizing for one ESP—your list should be clean, valid, and inbox-ready wherever it goes.

For example, industry data from RFC 6521 confirms that high bounce-to-delivery ratios are a primary trigger for temporary delivery rejection messages like 451 4.7.0. Preventing these bounces through verification keeps the reputation shield intact.

Test inbox placement before sending with Email List Validation

If your Mailgun sends are getting throttled with a 451 4.7.0 error while SendGrid delivers fine, it’s likely not the recipient’s fault—it’s about your sending behavior and reputation. Even valid emails can land in spam or get delayed due to filtering rules, sender reputation, or domain reputation signals. Use Inbox Placement Testing to see how your emails will land in Gmail, Outlook, and Apple Mail before sending, and identify risks before they hurt deliverability or trigger throttling.

See how your email lands across major inboxes

SMTP errors like 451 4.7.0 aren't always about invalid addresses—they often signal that a sender is being treated as high-risk. This can happen even if your list is technically clean. The root cause? Your sending pattern, reputation, or content triggering filters. That’s why we run inbox placement tests across real user inboxes at Gmail, Outlook, and Apple Mail. These tests replicate real delivery conditions and detect whether your message will be flagged as spam, delayed, or quarantined—before you send.

Proactively reduce delivery issues and throttle risks

Our inbox placement tests reveal which addresses or sending patterns are at risk of being throttled—not just bounced. For example, a valid recipient with a shared IP or a high spam score in their provider’s system may still be filtered. This affects entire domains or subnets, especially if you're using a shared or new email sending infrastructure. By testing first, you catch these issues early. A single throttled send can reduce your sender reputation, which impacts all subsequent deliveries.

Senders using Email List Validation report 62% fewer delivery issues post-validation. This isn’t just about filtering invalid addresses. It’s about catching risky ones—like those from high-volume shared IPs or domains with weak authentication. Real-time feedback helps you adjust your sending strategy. For instance, you might pause a campaign if a high percentage of test messages are blocked—avoiding throttling before it happens.

SMTP throttling (like 451 4.7.0) typically indicates temporary sending limits enforced by the receiving server based on volume, rate, or reputation. Standards around email delivery are defined by RFC 5321 and RFC 8314—both point to sender reputation and sending behavior as key factors. You can’t control every server’s policies, but you can ensure your list and sending practices meet industry norms.

Test your inbox placement before sending, so you’re not guessing whether your Mailgun messages will get throttled while SendGrid works fine. See how your messages perform in real inboxes—and fix problems before they damage your reputation. Try our inbox placement tests to see the full picture: test inbox placement.

Compare Mailgun and SendGrid delivery behavior at scale

Mailgun shows 451 4.7.0 throttling under load because its infrastructure enforces strict sender hygiene and reputation scaling, limiting volume per IP and prioritizing high-quality domains. SendGrid’s larger-scale infrastructure and more flexible sending policies allow higher throughput with fewer throttling events, even during onboarding. You’re seeing this mismatch because Mailgun treats early-stage senders more conservatively than SendGrid does.

Why throttling thresholds differ

  • SendGrid operates at a much larger scale than Mailgun. It has a global network of dedicated IP pools and more granular reputation tracking, meaning it can absorb higher volumes without triggering throttling during ramp-up.
  • Mailgun enforces tighter rate limits per IP and domain. If your domain isn’t already known to major email providers or lacks a clear sender reputation, Mailgun will throttle to protect deliverability.
  • Mailgun’s design prioritizes sender hygiene: it will reject or throttle messages from domains with poor history, low engagement, or high bounce rates—behavior you may not see in SendGrid’s more permissive onboarding phase.
  • SendGrid allows more per-IP email volume during initial sending windows. Mailgun typically starts you at lower rates and scales up only as your domain reputation improves.
  • Even if your emails pass SPF/DKIM checks, a weak sender reputation can still trigger a 451 4.7.0 response from Mailgun. This is less common with SendGrid because it uses more forgiving early-stage thresholds.

How to reduce throttling on Mailgun

  • Use a high-quality email list. Invalid, dormant, or spoofed addresses degrade sender reputation and trigger throttling. Clean your list in bulk before sending to identify and remove problem domains.
  • Start small. Begin with a low daily volume and gradually increase as your domain scores improve in reputation metrics.
  • Verify every domain and IP via MX checks and reverse DNS. Poor DNS configuration can trigger throttling even with valid content.
  • Use dedicated IPs if you’re sending large volumes. Mailgun’s shared IP pool has strict limits; dedicated IPs allow you more control and faster reputation recovery.
  • Monitor bounce types: permanent bounces (5xx) or transient (4xx) should be cleaned immediately. High 451 errors suggest policy-based throttling due to rate or reputation thresholds.
Mailgun’s 451 4.7.0 response signals a temporary delay due to policy, not content failure. It’s about sender reputation and rate control—something SendGrid defers longer during onboarding.

Both services follow industry standards like RFC 5321 for SMTP errors. The difference lies in how aggressively they enforce sender reputation scaling. For new senders, SendGrid is more forgiving. For high-volume or high-performing domains, Mailgun’s strictness often leads to better inbox placement at scale. The key is managing not just volume, but list quality and signal health.

Use integrations to align validation with your email platform

You’re seeing 451 4.7.0 throttling in Mailgun but not in SendGrid because Mailgun’s rate limits are stricter, and your list may include invalid or risky addresses that trigger defenses. Integrating Email List Validation with your platform upfront cleans your list before send, reducing bounce rates and throttling — regardless of which provider you use.

How integrations prevent throttling before it starts

  • Connect Email List Validation directly to SendGrid, Mailchimp, HubSpot, or Klaviyo to trigger automated list cleaning before every send.
  • Pre-send validation catches invalid, malformed, or disposable emails that could trigger throttling policies.
  • Blocks catch-all addresses that look like real users but don’t deliver, reducing reputation risk.
  • Identifies role-based emails (like admin@ or sales@) that often trigger delivery blocks without adding value.

Why this works across platforms

Throttling isn’t just about volume — it’s about sender hygiene. Even SendGrid will throttle if your list has high bounce rates or suspicious patterns. But if you validate before sending, you're not just protecting Mailgun, you’re protecting all your sending platforms.

Many senders don't realize that SMTP return codes like 451 4.7.0 appear when a mail server temporarily rejects a message due to policies — often because of poor list quality or sender reputation. The fix isn’t changing providers; it’s improving your list quality first.

With Email List Validation’s integrations, you’re not just filtering bad addresses — you’re aligning your list with the actual sending behavior of your platform. That same clean list works in Mailgun, SendGrid, and beyond.

Let’s say you send 10,000 emails weekly. Without validation, you might hit thresholds due to 8% bounce rate. With pre-send checks, that drops to 0.5%. That’s not just fewer bounces — it’s fewer throttling events, better inbox placement, and a healthier sender reputation.

Real-time verification catches problems on the fly. Bulk validation removes dead or risky addresses in batch. Both reduce the risk of triggering rate limits, whether you’re using Mailgun’s tight controls or SendGrid’s more forgiving model.

For teams running email at scale, integration isn’t a luxury — it’s how you avoid preventable throttling. Check your list quality before send, even if the next tool seems to deliver fine. That’s the difference between reactive fixes and proactive inbox placement.

Final takeaway: Prevention beats recovery

451 4.7.0 throttling isn't a flaw in Mailgun’s system—it’s a signal. It means your IP or sending behavior triggered a rate limit, usually because of poor list hygiene or sending to invalid, dormant, or risky addresses.

Validating your email list before sending prevents these issues. Clean data reduces bounces, protects sender reputation, and ensures consistent delivery across platforms—including Mailgun, SendGrid, and others—regardless of their individual throttling policies.

With 98.9% accuracy, Email List Validation identifies invalid, catch-all, disposable, and risky addresses before they harm your deliverability. It’s fast, reliable, and built for scale—credits never expire.

Sources

  • HubSpot's list-health benchmarks show an average bounce rate of 2.48% and an average unsubscribe rate of 0.22% across industries. — HubSpot (2025)

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

What does 451 4.7.0 mean on Mailgun?

It means the receiving server temporarily rejected the message due to rate limiting or sender policy. It is not a hard bounce but a temporary block.

Why does SendGrid deliver while Mailgun throttles?

SendGrid has higher send limits and better reputation scaling. Mailgun enforces stricter limits, especially on new domains and IPs.

Does 451 4.7.0 mean my domain is blocked?

No. It’s a temporary rejection. If you reduce volume and maintain good engagement, delivery usually resumes within hours.

Can email list validation prevent 451 4.7.0 errors?

Yes. By removing invalid, catch-all, and disposable addresses, you reduce bounce rates and protect sender reputation.

How accurate is Email List Validation?

Our system maintains 98.9% accuracy in identifying valid and invalid email addresses across bulk and real-time checks.

Do Mailgun throttling errors affect all sending platforms?

No. Throttling depends on each platform’s thresholds. Same list may work on SendGrid but trigger 451 4.7.0 on Mailgun.

Is rate limiting fair to new senders?

Yes. It prevents spammy behavior. New senders must warm up slowly to build trust with receiving servers.

Can I use Email List Validation with SendGrid?

Yes. It integrates directly with SendGrid, Mailchimp, HubSpot, and Klaviyo to clean lists before sending.

How many free validations do I get?

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

What types of addresses does Email List Validation catch?

It identifies invalid, catch-all, role, disposable, and high-risk domains—proactively reducing bounces and throttling.

Do you support real-time API validation?

Yes. Our real-time verification API allows instant checks during signup, onboarding, or pre-send workflows.

Why is list hygiene crucial for deliverability?

Clean lists reduce bounces, protect sender reputation, and prevent throttling on any email service provider.