What are Brevo SMTP relay bounce and complaint webhooks?

You send an email. It goes out through Brevo’s SMTP relay. Then—silence. No delivery. No open. No reply. But you don’t know why. Was it a bounced address? A spam complaint? Or a misconfigured server?

Here’s what you’re missing: real-time alerts. Brevo SMTP relay bounce and complaint webhooks send immediate notifications when something goes wrong—like when an email fails to deliver (bounce) or gets marked as spam (complaint). These events include the recipient address, status code, and event type, so you can act fast.

Without monitoring these webhooks, you risk sending to invalid addresses, spam traps, or blacklisted domains. That erodes your sender reputation, hurts deliverability, and can trigger domain-wide blocks. It’s like driving blind—eventually, you crash.

Key takeaways

  • Brevo SMTP relay webhooks deliver real-time alerts when emails bounce or are marked as spam.
  • Each webhook includes the recipient address, delivery status, and event type—critical for tracing and fixing problems.
  • Ignoring these events leads to sender reputation damage, domain blacklisting, and reduced inbox placement.

Why are webhooks from Brevo SMTP relay critical for deliverability?

You need Brevo SMTP relay webhooks because they deliver real-time alerts about bounces and complaints—two of the most damaging signals to your sender reputation. Gmail, Outlook, and Yahoo track these metrics closely: a bounce rate above 2% or complaint rate above 0.1% can trigger delivery throttling or outright blocking. Webhooks give you the earliest possible warning, letting you act before reputation damage becomes systemic.

How bounces and complaints hurt your sender reputation

Every bounce—whether hard (invalid address) or soft (temporary failure)—tells email providers that your messages aren’t reaching valid inboxes. A high rate of either signal poor list hygiene, increasing the odds your future emails are filtered or rejected.

Complaints are even more serious. When a recipient marks your email as spam, it directly harms your reputation with gatekeepers like Gmail and Microsoft. According to industry standards, even a few complaints per 1,000 emails can flag your domain as high-risk. The longer you ignore them, the deeper the damage.

Why real-time visibility matters

Without webhooks, you might only discover issues days later—after your emails are already blocked. But Brevo SMTP relay webhooks send data instantly when a bounce or complaint occurs. This allows you to react before your sender reputation is compromised.

For example, if a single domain starts generating repeated bounce notifications, you can quarantine it immediately. If a particular campaign triggers a spike in complaints, you can pause it and investigate the content or audience. The faster you respond, the better your long-term deliverability.

Webhooks are not a fix-all, but they are foundational. They turn reactive troubleshooting into proactive prevention. Tools like MailTester's bulk email verification help you avoid sending to risky addresses in the first place—catching many issues before they even get sent.

Let’s be clear: no system prevents every problem. But webhooks and verification together form the core of a resilient deliverability strategy. Use them to stay ahead of email providers’ thresholds, not behind them.

Learn more about how real-time verification and monitoring work together: MailTester integrations with platforms like Brevo help automate cleanup and improve inbox placement.

How do Brevo's bounce events differ from complaint events?

Bounce events mean the recipient server rejected your email during the SMTP handshake—usually due to a technical issue like an invalid address or full mailbox. Complaint events mean a recipient actively marked your message as spam, which is more serious because it reflects direct user intent and can hurt your sender reputation. Bounces are about delivery failure; complaints are about user experience and signal list quality problems.

Bounce Events: Technical Delivery Failures

Bounces happen during the SMTP transaction when a recipient server refuses to accept an email. This could be due to a typo in the address, a non-existent mailbox, or a temporary issue like a full inbox. They’re typically automatic and not reflective of the user’s personal preference—just a technical block.

For instance, a hard bounce from a non-existent domain means the email server never recognized the address. These are clear signals to remove the address from your list. The SMTP protocol (RFC 5321) defines these rejection codes, which Brevo’s relay uses to trigger bounce webhooks.

Complaint Events: User-Driven Spam Indicators

Complaints occur when a recipient marks your message as spam in their email client. This is a direct signal that the user didn’t want the email, whether it was irrelevant, unsolicited, or poorly targeted. Unlike bounces, complaints don’t reflect technical delivery issues—they reflect the recipient’s actual behavior.

According to Spamhaus, a single complaint can trigger increased scrutiny from email providers, especially if the rate is high relative to your volume. While a few bounces might be normal, multiple complaints can lead to throttling or blacklisting.

If you're seeing spikes in complaints, it’s a red flag: your list may include low-quality or unengaged recipients. Tools like MailTester’s bulk email verification can catch invalid or risky addresses before you send, reducing both bounce and complaint rates.

Common causes of Bounce and Complaint events in Brevo SMTP relay

You get bounces and complaints in Brevo SMTP relay when your emails hit invalid addresses, temporary delivery issues, or trigger spam filters due to poor content or unengaged recipients. Hard bounces come from non-existent or invalid email addresses—often old, role-based (like admin@ or sales@), or never verified. Soft bounces result from temporary server problems like full inboxes or timeouts, which can signal reliability issues over time. Spam complaints arise when recipients mark your messages as spam, usually because of over-messaging, low-quality content, or sending to subscribers who have opted out. These events hurt sender reputation and can lead to higher blocking or filtering.

Hard bounces: invalid or unreachable addresses

Hard bounces occur when an email address is permanently undeliverable—typically because it’s misspelled, doesn’t exist, or belongs to a domain that rejects mail. Role-based addresses like postmaster@ or info@ are especially risky; many are catch-alls, meaning they accept any message without verifying it’s valid. Brevo SMTP relay flags these as hard bounces so you don’t waste send attempts. Sending to unverified lists increases this risk. A single invalid address can harm your sender reputation over time, especially if it’s part of a larger pattern.

Let’s be clear: catching these before sending saves time, money, and deliverability. Tools like bulk email list verification check for these issues at scale, identifying dead or risky addresses so you can clean your list before sending. This is a proven way to reduce bounce rates and improve inbox placement with providers like Brevo.

Soft bounces and complaint triggers

Soft bounces are temporary. They happen when an inbox is full, the server is down, or a message is too large. While not a hard rejection, repeated soft bounces suggest sending to unreliable or slow recipients—often a sign of outdated or inactive data. Left unchecked, they erode sender reputation and can eventually lead to throttling or blocking.

Spam complaints are more serious. They happen when a recipient actively reports your email as spam. Common triggers include sending too frequently, using misleading subject lines, or targeting unengaged users. According to industry data from RFC 6653, sender reputation is heavily influenced by user feedback, and consistent complaints can result in immediate filtering or blocklist placement. Always ensure your content is relevant, your frequency is appropriate, and your audience is engaged.

If you're unsure whether an address is safe to send to, you can test it first with real-time email validation. This check reveals whether it’s likely to bounce or trigger spam complaints—before it happens.

Use case: How to act on Brevo bounce and complaint webhooks

When Brevo sends bounce or complaint webhooks, act immediately: log each event with recipient, timestamp, and type. Remove hard bounces instantly to protect sender reputation. Flag repeated complaints—these often point to list quality issues or content that triggers spam filters. Use a tool like MailTester to verify suspect addresses before re-adding them.

Step-by-step: Turning webhooks into action

  1. Set up a secure endpoint to receive Brevo webhooks. This should log event type (hard bounce, soft bounce, complaint), timestamp, and recipient email. Use this data to track delivery health and detect patterns. Tools like RFC 5321 define how mail servers communicate, and proper handling of delivery status codes is foundational to reliable email infrastructure.
  2. Automatically exclude hard bounces from future sends. A hard bounce means the address doesn’t exist or the domain is unreachable. Retrying these wastes resources and can hurt your sender reputation. Removing them at source prevents damage to your domain's deliverability over time.
  3. Monitor and alert on repeated complaints. If the same email or domain triggers complaints multiple times, it suggests either outdated addresses, content that feels spammy, or a list that isn’t properly cleaned. Investigate these cases—it’s often a sign your list hygiene needs work.
  4. Verify questionable addresses before re-adding. When an address keeps bouncing or gets reported, don’t assume it’s still valid. Use a real-time email verification tool to test its status. MailTester’s email checker validates syntax, domain existence, and inbox responsiveness without sending a message.

Why this matters beyond just list cleanup

Brevo’s webhooks are only useful if you act on them. Ignoring complaints leads to blocklisting. Ignoring hard bounces risks blacklisting by ISPs. You’re not just protecting your deliverability—you’re respecting the user’s inbox. According to Spamhaus, domains with high complaint rates are more likely to be added to real-time blocklists.

Use your logs to run monthly health checks. Compare bounce types across campaigns. If one campaign shows higher soft bounce rates, it may point to content or timing issues. If a domain is consistently problematic, avoid it entirely. Over time, this process turns raw data into cleaner lists, higher inbox placement, and better engagement.

How MailTester prevents bounces and complaints before they happen

You can drastically reduce bounces and complaints on Brevo SMTP relay by verifying email addresses before sending—MailTester’s real-time API and bulk verification catch invalid, catch-all, role, and disposable emails with 98.9% accuracy, so you only send to addresses that are likely to deliver and engage. This keeps your sender reputation strong and inbox placement high.

Real-time validation stops problems at the source

Let’s say you’re about to send a campaign via Brevo. Instead of guessing if an address is valid, your system can run it through MailTester’s real-time verification API—in under 100ms. The API checks for syntax errors, whether the domain exists, if it’s a catch-all mailbox (which can’t be validated), and if it’s a role account (like admin@ or sales@) or a disposable email (like 10minutemail.com). These are early red flags before you ever hit "send."

Bulk verification cleans your list before the send

For larger campaigns, use MailTester’s bulk email list verification tool to scan thousands of addresses at once. It identifies and flags addresses that will likely hard bounce—up to 90% of them—before they ever hit Brevo’s SMTP relay. That means fewer bounces, lower complaint rates, and less risk of being flagged by Brevo or inbox providers like Gmail and Outlook. Industry standards show that clean lists significantly improve deliverability and reduce strain on sending infrastructure.

It’s not just about avoiding hard bounces. Role accounts and disposable domains often lead to poor engagement and spikes in complaints—especially if a user never intended to receive your email. By removing these addresses early, you maintain a healthy sender reputation, which is crucial for consistent inbox placement. This is why platforms like Brevo track sender reputation and enforce anti-abuse policies.

For more granular testing, you can also use inbox placement testing to see how your message looks inside real inboxes—before sending to your full list. This helps you avoid sending messages that trigger spam filters or user complaints.

According to industry guidance from RFC 5321, SMTP servers should reject invalid recipients early. By verifying addresses before sending, you align with standard best practices and support a cleaner, more reliable email ecosystem.

Integrating MailTester with Brevo to proactively reduce bounces

You can prevent bounces before they happen by verifying every email address added to Brevo—whether through forms, imports, or APIs—using MailTester’s real-time verification. This stops invalid, outdated, or risky addresses from polluting your lists and hurting deliverability. By validating at ingestion, you reduce bounce rates, avoid reputation damage, and improve inbox placement. This is the simplest, most effective defense against failed sends.

Set up real-time verification at list entry

  1. Use MailTester’s API to validate addresses as they’re added to Brevo. When a user signs up via a form or API, call MailTester’s verification API to check the email against SMTP, DNS, and domain reputation signals. A valid response means it’s likely deliverable—skip invalid entries before they land in your Brevo list.
  2. Integrate via Zapier, Make, or direct API calls. Tools like Zapier and Make can trigger MailTester checks on form submissions or data syncs. For full automation, connect directly using HTTP requests. This ensures only valid addresses make it into Brevo, cutting down the risk of hard bounces and complaints.
  3. Block or flag invalid emails in real time. Use the API response to decide: automatically reject invalid emails or mark them as “risky” for manual review. This avoids sending to addresses that are likely to bounce or generate spam complaints.

Run bulk verification on existing lists

  1. Pre-send cleanup with bulk verification. Before launching campaigns—especially large ones—upload your existing Brevo list to MailTester’s bulk verification tool. The system checks each address in real-time using up-to-date sender reputation and SMTP infrastructure data. You’ll get a detailed report identifying invalid, catch-all, and risky addresses.
  2. Remove or segment problematic addresses. Use the results to prune invalid emails, separate questionable addresses for revalidation, and retain only confirmed deliverable ones. This directly lowers your bounce rate, which is a key signal to inbox providers.
  3. Monitor for ongoing list decay. Even clean lists degrade. Regularly check new additions and remove old ones that haven’t engaged. This keeps your sender reputation strong and maintains good standing with platforms like Gmail, Outlook, and others.

MailTester’s 98.9% accuracy is built on live SMTP checks, MX validation, and reputation monitoring. It’s an industry-standard practice to verify addresses before sending—and platforms like Spamhaus and RFC 5321 confirm that sender reputation is tied directly to list hygiene. This isn't optional—it's foundational.

Why traditional list cleaning isn't enough—email verification beats post-send filtering

You can’t fix a damaged sender reputation after the first 100 bounces or complaints. Waiting for feedback from spam traps or ISPs like Gmail and Outlook means your domain's credibility is already eroding. Email verification with MailTester stops invalid addresses before they ever reach your sending platform, eliminating bounces at the source and protecting your long-term deliverability.

Reactive filtering fails you when reputation is on the line

The moment an email bounces or gets reported as spam, your sender reputation takes a hit—sometimes permanently. Most ESPs, including Brevo, send bounces and complaints via webhooks, but those are signals after the damage is done. By the time you act, your domain may have already been flagged by major providers like Microsoft or Google for poor engagement or high error rates.

Let’s be clear: relying on post-send filtering is like locking the barn door after the horses are gone. You’re not preventing issues—you're managing the fallout. The real fix is not waiting for signals, but stopping bad addresses before they ever load into your send queue.

Verification stops invalid emails before they matter

Email verification with MailTester validates each address in real time—from syntax and domain existence to inbox activity and role account detection—before you send. The result? A list where only valid, active addresses remain. This isn’t just about lowering bounce rates. It’s about preserving your domain's trustworthiness across the board.

Studies show that senders who maintain low bounce rates consistently see better inbox placement, especially with aggressive filters like Gmail’s. You can’t control everything, but you can control where your emails start. Using the MailTester bulk verification tool or real-time API gives you full control over list hygiene, reducing waste and improving engagement from day one.

MailTester’s accuracy is backed by technical checks—SMTP validation, MX probing, and role account detection—without storing your data. The result is clean, verified data that’s ready to send, whether you're using Brevo, SendGrid, or another platform. You’re not just filtering out noise—you're building a reputation that lasts. With 100 free verifications to start, you can test the difference without a commitment.

Setting up real-time inbox placement testing with MailTester

You can test how your email will land in real inboxes before sending by using MailTester’s inbox placement tool. It sends test messages to Gmail, Yahoo, Outlook, and other major providers to reveal whether your content, headers, or branding triggers spam filters. Combine this with pre-send verification to ensure you’re only sending to valid, deliverable addresses—reducing bounces, complaints, and damage to sender reputation.

Run inbox placement tests before every campaign

  1. Send a test email through MailTester’s inbox placement tool at https://mailtester.com/inbox-tester/. You’ll send a fully rendered version of your email to real user inboxes across Gmail, Yahoo, Outlook, and others. No guesswork—just real-world results.
  2. Review the outcome report. The tool shows whether your message landed in the inbox, spam folder, or was blocked. It flags issues like suspicious links, poor sender reputation, or formatting problems that could trigger filters. This is the same data major ESPs use to decide inbox placement.
  3. Adjust content or headers based on feedback. If your test lands in spam, examine factors like image-to-text ratio, sender domain alignment, or link domains. Fixing these changes how email providers interpret your message.
  4. Use MailTester’s bulk verification to pre-screen your list at https://mailtester.com/email-list-verify/. This ensures only valid, active addresses are included—eliminating hard bounces and reducing the risk of spam complaints.
  5. Integrate verification into your workflow. Use the real-time API or embed verification in your CRM, marketing platform, or mailing service via integrations like Mailchimp, HubSpot, or SendGrid.

Mix inbox testing with real-time validation for full protection

You’re not just checking if an email exists—you’re checking if it will reach the inbox. Tools like Brevo SMTP relay can send emails to valid addresses, but if the sender reputation or content is poor, those messages still end up in spam. MailTester’s approach combines validity checks with delivery simulation, giving you a fuller picture.

For example, an address might be real but a catch-all or role-based (like admin@ or support@). These are high-risk for complaints and bounces. MailTester identifies and flags these with a risky verdict, so you know not to send to them. This is a common issue with role-based email addresses, which are often ignored or reported as spam.

By combining inbox placement testing with pre-send verification, you reduce the risk of delivery failure, protect sender reputation, and avoid the long-term harm of being flagged by filters or blacklisted by providers. This is a standard practice among high-volume senders who prioritize sustainable deliverability.

“The best campaigns don’t just send—they land.”

MailTester’s solution is built on the principle that real testing beats assumptions. You don’t need to wait for a delivery failure to learn what works. Test it first, fix it early, send with confidence.

Key takeaway: Bounce and complaint webhooks are only useful if paired with proactive list hygiene

You can’t rely solely on Brevo’s SMTP relay bounce and complaint webhooks to maintain deliverability. They react to problems after they happen. The real defense is validating every email address before sending—using tools like MailTester’s 98.9% accurate verification—so you stop bad addresses from ever entering your send queue. Without proactive checks, webhooks are just fire alarms instead of prevention.

Webhooks alone don't stop bad sends

Bounce and complaint webhooks tell you when something broke, but not why it broke. A single hard bounce from a dead address won’t hurt your sender reputation today—but 500 such bounces a week will. Waiting for those signals means you’ve already sent to invalid emails, wasted bandwidth, and risked blacklists. The industry standard is to reduce the number of undeliverable messages by design, not response.

Pre-emptive verification is the only sustainable approach

Let’s be honest: you can’t clean a list after every send and stay ahead of deliverability decay. The only way to sustain high inbox placement over time is to verify every address before it hits your mail server. Real-time validation tools don’t just catch typos—they flag disposable domains, role accounts, and catch-all setups that silently sabotage your sender reputation. MailTester’s 98.9% accuracy rate, backed by checks on SMTP, MX, DNS, and mailbox behavior, removes guesswork.

And because purchased credits never expire, you’re not pressured into rushed or inefficient verification cycles. You can validate 100 emails today, 1000 next week, and never lose your investment. This consistency is essential—especially when working with tools like Brevo, where the cost of a single high-volume send to invalid addresses compounds quickly.

Think of it like this: webhooks are your smoke detector. Verification is your fireproofing. You wouldn’t depend only on the alarm to protect your home. Similarly, don’t rely only on Brevo’s webhooks. For reliable, long-term deliverability, you need bulk email verification as your first line of defense.

Start with MailTester’s 100 free verifications to clean your Brevo list today

Every bounce and complaint in your Brevo SMTP relay stats is a sign of list decay. Left unchecked, they damage sender reputation and hurt inbox placement.

MailTester lets you verify email addresses in bulk or in real time—no setup, no risk. Use the 100 free credits to audit your current list or test new subscriber capture flows before they reach Brevo.

Purchased credits never expire. You’re not rushed to use them. This means you can plan your list hygiene at your own pace, with confidence.

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

Frequently asked questions

What does a Brevo SMTP relay bounce webhook mean?

It means the recipient’s email server rejected the message during delivery. Hard bounces indicate invalid addresses; soft bounces may mean temporary issues.

What triggers a Brevo complaint webhook?

A recipient manually reports your email as spam. This indicates poor engagement or content alignment and harms sender reputation.

How quickly do Brevo webhooks return bounce or complaint events?

Events are sent in real time—usually within seconds of delivery failure or complaint submission.

Can I receive Brevo bounce and complaint webhooks without a dedicated endpoint?

No. You need a server or third-party integration (like Zapier) to receive and process the webhook data.

Does MailTester work with Brevo's SMTP relay?

Yes. MailTester can be used before sending via Brevo to verify addresses, reducing bounces and complaints at the source.

Why should I use MailTester instead of waiting for Brevo webhooks to detect bad emails?

Because MailTester stops invalid addresses before you send—reactive monitoring is too late to prevent damage.

What types of email addresses does MailTester detect as invalid?

MailTester identifies invalid, catch-all, disposable, and role-based addresses with 98.9% accuracy.

Can I verify emails in bulk before uploading to Brevo?

Yes. Use MailTester’s bulk verification capability to clean your list instantly before sending via Brevo.

How do I set up MailTester’s API with Brevo?

Use the MailTester API to validate addresses in real time during list ingestion, or run bulk checks and only import verified addresses.

What if my Brevo list still has high bounce rates after using MailTester?

Double-check your content, frequency, and engagement levels—MailTester fixes list quality, but poor sending practices cause remaining issues.

Are MailTester credits permanent?

Yes. Purchased credits never expire, giving you long-term flexibility in email verification.

Does MailTester support role-based email addresses like admin@ or sales@?

Yes. MailTester flags role addresses as high-risk, helping you avoid sending to them unless intentional.