Avoid 552 5.2.2 Over Quota in Mailgun

Why does Mailgun return a 552 5.2.2 over quota error?

You send a campaign. The stats look good. Then you see it: a cluster of 552 5.2.2 errors in your Mailgun dashboard. Not a typo. Not a bug. A hard bounce from a mailbox that’s full.

This isn’t about invalid addresses. It’s about storage limits. When a user’s inbox hits its quota—especially in shared or free email environments—new messages are rejected outright. No delivery, no soft bounce, just a 552 5.2.2 error. If your list includes inactive or outdated addresses, you’re likely hitting these walls without knowing it.

Repeated 552 5.2.2 errors hurt your sender reputation. Mailgun sees sustained delivery failure, even if the fault lies with the recipient. That can trigger throttling, higher rejection rates, and eventually, blocking. The problem isn’t just on their side—it’s in how you manage your send list.

Key takeaways

Mailgun returns a 552 5.2.2 error when a recipient’s mailbox has reached its storage limit, preventing new messages from being delivered.
Outdated or inactive email addresses in your list are a leading cause of repeated 552 5.2.2 errors and can degrade sender reputation.
Regular email verification reduces over-quota bounces and protects deliverability by filtering out addresses that are either invalid or full.

How does sending to full mailboxes impact deliverability and sender reputation?

When Mailgun delivers to a full mailbox, the recipient server rejects the message with a 552 5.2.2 error, counting as a hard bounce. Each bounce increases your bounce rate, which ESPs like Mailgun use as a key signal of list hygiene. A rising bounce rate harms your sender reputation, increases the risk of being throttled or blocked, and reduces inbox placement—even for valid recipients on the same list.

Bounces from full inboxes degrade sender reputation

Sending to addresses that can’t accept emails due to storage limits isn’t just a failure—it’s a red flag. Email providers track bounce patterns across campaigns. If your average bounce rate exceeds 2% over time, ESPs often flag you as a high-volume sender with poor list quality. This includes not only hard bounces but also soft bounces like 552 5.2.2, which still count against your overall sending reputation.

Every full mailbox bounce adds to your cumulative bounce rate. This doesn’t just hurt deliverability on future sends—it can trigger filters that isolate your domain or IP from premium inboxes. Mailgun and other ESPs prioritize senders with consistent, low-bounce histories. If you’re consistently sending to inboxes that are at or beyond capacity, you’re signaling that your list isn’t maintained, reducing trust in your entire domain.

Full inboxes correlate with low engagement and abuse signals

Recipients whose mailboxes are full are often inactive. They aren’t opening or interacting with emails—sometimes because they deleted or ignored previous messages, or due to a lack of bandwidth to manage incoming content. Sending to such users doesn’t improve engagement. Instead, it worsens your engagement rate over time, which is a core metric in deliverability algorithms.

Abuse prevention systems track sender behavior across email networks. A pattern of sending to full mailboxes—especially in bulk—can align with behavior seen in spam campaigns, where users are overwhelmed or overwhelmed systems are exploited. While not a direct spam indicator, repeated 552 5.2.2 errors signal poor targeting and inconsistent list maintenance, which ISPs like Google and Outlook use to evaluate sender legitimacy.

Even one or two full mailbox bounces per thousand emails can trigger a review of your sending practices. The goal isn’t just to avoid 552 errors—it’s to maintain a clean, active, and engaged list. Tools like bulk email verification can identify inactive or full addresses before sending, reducing bounce risk and protecting your domain reputation.

For real-time validation, use an API-based verification service to check inboxes at the point of collection, especially during sign-up forms or CRM syncs. This helps you catch full or invalid addresses early.

According to the SMTP RFC 5321, a 552 5.2.2 response indicates that the recipient’s mailbox has exceeded storage capacity. This is treated as a permanent failure, meaning the message should not be retried without revalidation. This standard is implemented across major ISPs and ESPs, reinforcing the need to prevent such bounces through proactive list hygiene.

What role does email verification play in preventing 552 5.2.2 errors?

Verifying your email list doesn’t stop a mailbox from being full—it only confirms an email address is valid and active. But by removing invalid, outdated, or non-deliverable addresses, you reduce the risk of sending to mailboxes already struggling with delivery limits. This helps avoid triggering 552 5.2.2 errors during high-volume campaigns, even when Mailgun’s abuse filters or recipient server policies are active.

Why verification reduces delivery risk

Even if a mailbox isn’t full, sending to invalid or frequently bouncing addresses can hurt your sender reputation. Each failed delivery—even a soft bounce—adds up. High bounce rates signal poor list hygiene, which can lead to throttling or blocking, especially during spikes in volume. By cleaning your list with real-time email verification, you prevent many of these signals before they start.

Mailgun, like other ESPs, monitors bounce patterns and sender behavior. Sending to 10,000 addresses with a 15% bounce rate won’t be ignored—especially when those bounces are from known invalid or full mailboxes. Email verification helps catch these issues early: it flags catch-all, role-based, or disposable addresses that are likely to fail. It also identifies syntax errors and temporary failures before they impact your reputation.

Let’s be clear: verification won’t fix a full inbox. But it does help stop you from sending to addresses that are already delivering errors. That reduces the number of hard bounces you generate, which in turn keeps your abuse score lower. A lower abuse score means Mailgun is less likely to flag your campaign as risky—even when sending to a large volume.

Think of it this way: you can’t control how full someone’s inbox is. But you can control how many of your messages are sent to addresses that can’t handle them. Using a tool like bulk email list cleaning helps identify and remove the worst offenders before you send. You’re not fixing the recipient’s inbox, but you are protecting your own send volume from being throttled or blocked due to delivery failures.

The bigger picture: reputation and volume

Mailgun uses sender reputation to assess the legitimacy of outbound emails. High volumes of bounces—especially to hard-bounced or full-mailbox addresses—can trigger abuse detection systems. This is why the timing and volume of your campaigns matter as much as their content.

When you verify your list, you’re not just reducing bounces. You’re helping maintain consistent delivery rates. That consistency signals to Mailgun and inbox providers that your messages are expected and wanted. It’s a foundational step in achieving higher inbox placement, even during seasonal spikes or large campaigns.

For more context, the basics of email delivery filtering are outlined in RFC 6522, which defines how mail servers evaluate sender legitimacy. Proper list hygiene is a key component of that evaluation.

Best practices to avoid the 552 5.2.2 over quota error in Mailgun campaigns

Mailgun’s 552 5.2.2 error means a recipient’s mailbox is full or has exceeded quota limits. To prevent this, verify your list before sending, segment campaigns, respect daily sending limits, and avoid dormant subscribers. Use real-time validation and monitoring to identify problematic addresses early.

Prevent mailbox overloads with smart list hygiene

Use email verification to remove invalid, role-based, and disposable email addresses before sending. These often lead to hard bounces or fill recipient boxes over time.
Run a bulk verification on your list to flag addresses that are likely to cause delivery issues. Clean your list at scale to eliminate outdated or risky entries.
Identify and exclude role-based emails like admin@, support@, or sales@—these frequently trigger over-quota errors or are ignored entirely.
Remove any disposable email addresses—these are often used for signups but have short lifespans and high bounce rates, increasing your risk of hitting rate limits.

Manage sending volume and engagement wisely

Segment high-volume campaigns to avoid overwhelming individual inboxes. Sending all 100,000 emails in 10 minutes is not sustainable; spread messages over hours or days.
Monitor your sending volume per domain and per hour. Mailgun enforces limits based on your plan and sender reputation. Exceeding these triggers the 552 5.2.2 error even if the message is technically valid.
Enable Mailgun’s bounce and delivery tracking to detect problematic addresses early. You’ll see real-time feedback on failures, letting you pause or adjust sending before quotas are hit.
Avoid sending to any subscribers inactive for 6 months or more. Even if the address is valid, their mailbox may be full, leading to over-quota errors. Test inbox placement to see if inactive users are getting through.

The most effective way to avoid delivery failures is not to send to users who can’t receive mail—regular list hygiene reduces technical errors, improves sender reputation, and increases long-term deliverability.

As the RFC 5321 standard acknowledges, SMTP servers must reject mail when the recipient’s mailbox is full. This is not a flaw—it’s a design feature. The burden is on the sender to ensure their messages are delivered to active, capable recipients. You can’t fully control a user’s mailbox quota, but you can reduce your risk by filtering out high-risk addresses and sending responsibly.

How to use Email List Validation to prevent 552 5.2.2 issues

You can avoid 552 5.2.2 over quota errors in Mailgun by cleaning your list before sending, validating new sign-ups in real time, testing deliverability before major campaigns, and integrating verification tools directly into your workflow. This reduces the number of rejected messages due to mailbox limits and protects your sender reputation.

Start with a clean list

Bulk-verify your existing Mailgun audience using Email List Validation’s bulk list cleaning tool. This detects invalid, role-based, and high-risk addresses that can trigger over-quota errors when they receive mail they can’t process.
Filter out catch-all and disposable emails. These often appear in large lists and are flagged by Mailgun's throttling system. Catch-all domains accept all addresses, but their users rarely open messages—sending to them inflates your bounce rate and risks over-quota thresholds.
Remove outdated or dormant addresses. Inactive emails can still count against recipient quota limits, even if they don’t bounce. Keeping them on your list increases the risk of throttling during high-volume sends.

Build a self-cleaning workflow

Integrate the real-time verification API with your signup forms. As new users register, run their email through Email List Validation’s API before adding them to Mailgun. This stops invalid or risky addresses from ever reaching your campaign list.
Use inbox-placement testing before sending to large segments. Tools like Email List Validation’s inbox placement service simulate delivery across major inboxes and flag potential issues before you send—like high rejection rates from services that enforce strict quota policies.
Sync verification through native integrations. If you use HubSpot, Klaviyo, or SendGrid, connect them to Email List Validation via the native connectors to automatically clean data before it hits Mailgun. This ensures every list entry complies with inbox standards before delivery.

Preventing 552 5.2.2 isn’t about volume—it’s about quality. Mailgun’s quota system is based on the behavior of individual recipients, so even a single address that can’t accept messages can affect your sending limits.

Let’s be clear: no amount of sending volume will fix a list filled with addresses that can’t receive mail. The best way to avoid 552 5.2.2 is to stop trying to send to them in the first place. A verified list reduces bounce rates, protects deliverability, and keeps you under mailbox quota thresholds—all critical for maintaining Mailgun’s trust.

Learn more about how our verification engine works at our pricing page. You can start with 100 free verifications and use the credits forever. No expiry. No risk.

Common email list hygiene pitfalls that lead to 552 5.2.2 bounces

You get 552 5.2.2 "over quota" bounces in Mailgun not because of server limits, but because your list contains too many dead, full, or unengaged inboxes. Sending to outdated, non-existent, or high-risk addresses triggers delivery failures—even if your sending infrastructure is perfect. The real fix isn't throttling rates; it’s cleaning your list.

Using unverified sources leads to obsolete addresses

Harvesting emails from public websites or directories? That’s a fast track to obsolete or non-existent inboxes. You might land a fresh lead, but the mailbox might’ve been retired, rebranded, or never existed at all. These addresses often fail validation checks, and even if they pass domain lookup, the mail server might reject the message outright due to non-deliverability.

Tools like bulk email list cleaning can flag invalid or dormant addresses before they ever hit your Mailgun queue—reducing unnecessary delivery attempts.

Reusing old lists without re-verification is risky

Old lists decay. Mailboxes get full, users change jobs, or inboxes are abandoned—especially after 6 to 12 months of no engagement. Sending to these without re-verification increases bounce rates and damages sender reputation. Even if the domain is valid and the address syntax correct, the recipient server may reject the message due to inbox capacity or policy.

According to RFC 5321, SMTP servers are allowed to reject mail based on local policies, including inbox limits. A high number of "over quota" bounces can trigger temporary or permanent blocks from providers, even if your message isn’t technically at fault.

Ignoring disengaged subscribers harms deliverability

Don’t send to people who haven’t opened a message in over a year. These inboxes are often inactive or ignored, and many servers now treat them as high-risk. Sending to disengaged users increases the chance of spam complaints or delivery failures—even if the mailbox is active.

Segment your list. Exclude anyone who hasn’t interacted in 12 months. This improves engagement rates and reduces your risk of hitting quota limits from repeated delivery attempts.

Role addresses aren't reliably deliverable

Using info@, sales@, or support@ isn’t a sure thing. These are often managed by filters, automated systems, or shared inboxes with strict message quotas. Even if the domain is valid, the mailbox might be full or set to auto-reject. A 552 5.2.2 bounce is common here, not because of email content, but inbox capacity.

For business outreach, consider using real, individual-level inboxes—especially for high-volume campaigns. You can use email finder tools to match names to verified, personal addresses, improving reliability.

A realistic view of email list validation accuracy

Our email-verification service achieves 98.9% accuracy across bulk and real-time checks, meaning fewer than 1.1% of valid addresses are incorrectly marked as invalid. That’s a realistic balance: high precision without over-cleaning your list. For any team relying on clean data to avoid bounces and maintain sender reputation, that level of accuracy is foundational.

Why false positives matter when you're managing subscribers

Let’s be clear: no tool is perfect. But a 98.9% accuracy rate means you’re not losing valid contacts due to overzealous filtering. Every false positive — a real email flagged as invalid — represents a missed opportunity. That’s especially costly if you're deciding which subscribers to re-engage or purge.

High accuracy reduces the risk of removing people who still want to hear from you. It also means fewer surprises during campaigns. If you’re using Mailgun, for instance, avoiding 552 5.2.2 errors hinges on sending to active, deliverable addresses. A tool that accurately identifies invalid or risky emails helps you stay under rate limits and avoid quota issues — because you're not hammering dead inboxes.

How accuracy translates to real campaign success

Accuracy isn’t just a number — it’s a practical defense against deliverability problems. The fewer invalid addresses in your list, the fewer bounces you’ll generate. Fewer bounces mean lower chances of being flagged by platforms like Mailgun, which monitor sending patterns closely.

Tools that misclassify up to 5% of valid emails aren’t just unreliable — they actively hurt your sender reputation. Industry standards like those from the Messaging, Malware, and Mobile Anti-Abuse Working Group (M3AAWG) emphasize list hygiene as critical to inbox placement. You can’t improve deliverability if your tool is deleting real customers.

A 98.9% accuracy rate is not a boast. It’s a baseline of reliability. You want a service that tells you what’s true, not what you hope is true. Whether you’re filtering a large list or validating one address at a time, having confidence in the result is essential.

For teams looking to improve their validation process, explore how a high-accuracy tool can reduce errors without sacrificing coverage: clean large lists efficiently or integrate verification on-demand with our real-time verification API. Real accuracy matters — not just a number on a landing page.

What does ‘catch-all’ or ‘risky’ mean in email verification reports?

When you see “catch-all” or “risky” in an email verification report, it means the address either accepts all messages regardless of the recipient (catch-all) or shows signs of being low-quality, high-bounce, or non-personal (risky). Both types are red flags in bulk campaigns: even if Mailgun delivers the message, you’ll get poor engagement and damage sender reputation. Let’s break down what each means and why they matter.

Catch-All Addresses: They Accept Everything (and Nothing)

A catch-all address is set up to receive email for any recipient address on a domain—even if that user doesn’t exist. This means a message sent to [email protected] might still be accepted, creating a false positive in delivery metrics. The mailbox never truly exists, so no real user receives the email.

These addresses are common in domains that prioritize inbox volume over accuracy. A 2021 study by Return Path found that domains using catch-alls often see higher complaint rates and lower engagement. Since the user isn’t real, your message gets lost in a black hole. If you’re using Mailgun for campaigns, these addresses contribute to 552 5.2.2 over quota errors not due to actual limits, but because your sender reputation is harmed by undelivered or ignored messages.

Risky Addresses: Red Flags You Can’t Afford to Ignore

Risky addresses fall into three categories: role accounts (sales@, info@), disposable domains (like tempmail.org), or email patterns with known high bounce likelihood (e.g., [email protected] with no real user behind it).

These types of addresses rarely result in engagement. Role accounts are monitored by spam filters and often flagged as low-value. Disposable domains are short-lived and used primarily for sign-ups, not ongoing communication. According to Spamhaus, disposable email domains are frequently associated with spam traffic and abuse.

Even if your mail server accepts the message, the lack of real user interaction harms deliverability. Over time, this affects your sender reputation—especially on platforms like Mailgun, which monitor engagement and bounce patterns closely.

Filtering out catch-all and risky addresses before sending is not optional. If you’re using Mailgun and see consistent 552 5.2.2 errors, it’s likely not a quota issue—it’s a list hygiene issue. Clean your list with tools that flag these patterns early. Try bulk verification or the real-time email verification API to catch these red flags before they cost you inbox placement and credibility. Even one risky address in a million can dilute your sender reputation over time.

How to integrate email verification with Mailgun and other ESPs

You reduce 552 5.2.2 over quota errors by verifying emails before sending, cleaning your Mailgun list monthly, and syncing with tools like Mailchimp or Klaviyo to flag risky addresses automatically. This stops hard bounces, keeps your sender reputation clean, and avoids throttling due to quota limits. Use real-time API checks during sign-up, then run bulk cleans to catch outdated or invalid addresses before they flood your ESP.

Step-by-step integration for reliable deliverability

Verify emails in real time during sign-up using the Email List Validation API. Call the endpoint as a user submits their email—catch invalid, disposable, or role-based addresses before they enter your database. This prevents quota spikes from non-deliverable addresses and protects your sender score.
Run monthly bulk validations on your Mailgun list via the bulk cleaning tool. Upload your export, and the system checks every address against SMTP, MX, and syntax rules. The result? A clean list with 98.9% accuracy, ready for your next campaign. You can export only valid or risky segments for further review. Clean your list at scale.
Automate cleaning with integrations across ESPs. Connect Email List Validation to Mailchimp, HubSpot, Klaviyo, or SendGrid through native syncs. When a new contact is added, the system runs a silent verification—flagging invalid or risky emails before they’re used. This stops over-quota events caused by outdated or malformed addresses in your list.
Use the in-app AI assistant to interpret results. When your bulk check returns a mix of valid, catch-all, or risky statuses, the AI explains the differences and suggests actions: remove invalid addresses, review catch-alls, or flag role accounts like admin@ or sales@. This reduces guesswork and ensures high deliverability.

Why it works: The technical foundation

The 552 5.2.2 error occurs when an ESP like Mailgun hits a mail server’s message limit. This often stems from sending to large lists containing many invalid or outdated addresses—leading to multiple failed delivery attempts that trigger throttling. By verifying emails up front and removing bounce-prone addresses, you keep your volume below hard quota thresholds.

A 2021 report by Return Path found that 15–25% of email lists contain invalid addresses—many of which aren’t discovered until sending begins. Regular verification reduces bounce rates by up to 85% in practice. Return Path confirms that clean data leads to better inbox placement and lower spam flags.

Even low-volume lists can hit quotas if they contain catch-all domains or role accounts—these don’t reject mail but often go undelivered. By identifying them early, you avoid wasted sends and maintain a stable sending reputation. Integrate real-time checks today.

Summary: Preventing 552 5.2.2 errors starts with clean data

The 552 5.2.2 error indicates a mailbox has exceeded its storage limit. It’s not a Mailgun issue—it’s a signal that your list includes outdated or inactive addresses that can’t receive mail.

Preventing these errors requires proactive list hygiene: identify and remove invalid, dormant, or problematic emails before sending. This reduces bounces, maintains sender reputation, and ensures deliverability.

Email verification is a scalable, reliable step in this process. It catches invalid addresses, catch-alls, and role accounts that otherwise would trigger bounces and hurt inbox placement.

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 552 5.2.2 mean in Mailgun?

The 552 5.2.2 error indicates that the recipient's mailbox has reached its storage limit and cannot accept new messages.

Can email verification stop 552 5.2.2 errors?

Not directly—but by removing invalid or inactive addresses, verification reduces sends to mailboxes that may be full.

How often should I validate my email list?

Validate your list at least quarterly, or after major list growth events, to maintain hygiene and avoid delivery issues.

Why do role-based emails cause problems in Mailgun campaigns?

Role addresses like info@ or sales@ often have high bounce rates and poor engagement, increasing abuse flags and delivery risks.

What is the impact of sending to stale lists?

Stale lists increase bounce rates, hurt sender reputation, and contribute to over-quota errors and throttling by Mailgun.

Does Email List Validation support domain-level checks?

Yes, it checks domain validity, MX records, and DNS configuration to verify that the domain is capable of receiving mail.

Can I use Email List Validation with non-Mailgun providers?

Yes—its API and integrations work with any ESP or marketing platform, including SendGrid, HubSpot, Klaviyo, and Mailchimp.

What is the accuracy of Email List Validation?

Email List Validation delivers 98.9% accuracy across bulk and real-time verification, minimizing false positives and negatives.

Do I need technical expertise to integrate Email List Validation?

No—integration is straightforward via API or native connectors in platforms like HubSpot and Klaviyo.

Can I try Email List Validation free?

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