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No Bounce Handling Causing Sender Reputation Damage

Your application continues to send emails to addresses that have previously bounced (invalid addresses, full mailboxes, blocked domains). This damages your sender reputation score, causing email providers to throttle or block all your emails, including those to valid recipients.

Without bounce handling, your deliverability degrades over time. Emails that used to reach inboxes start landing in spam, then stop arriving at all. By the time you notice, your sending domain may be blacklisted and recovery takes weeks.

The issue compounds because every failed delivery attempt further damages your reputation, creating a downward spiral that affects all your users, not just the ones with invalid addresses.

Error Messages You Might See

550 5.1.1 User unknown 552 Mailbox full Email suppressed: previously bounced Sender reputation score: critical Domain blacklisted by Spamhaus
550 5.1.1 User unknown552 Mailbox fullEmail suppressed: previously bouncedSender reputation score: criticalDomain blacklisted by Spamhaus

Common Causes

  • No webhook listener for bounces — The email service reports bounces via webhooks but the app has no endpoint to receive them
  • Bounced addresses not flagged — Even if bounce notifications are received, the app doesn't mark addresses as invalid in the database
  • No distinction between hard and soft bounces — Hard bounces (invalid address) should permanently stop sending, soft bounces (full mailbox) should retry then stop
  • Suppression list not checked before sending — The app doesn't check a suppression list before attempting to send each email
  • Catch-all sending to user-provided addresses — Newsletter or notification features send to any address without validation

How to Fix It

  1. Set up bounce webhook endpoint — Create an endpoint to receive bounce notifications from your email provider (SendGrid Event Webhook, Mailgun webhooks, SES notifications)
  2. Maintain a suppression list — Store bounced addresses in a database table and check it before every send
  3. Handle bounce types differently — Permanently suppress hard bounces immediately. Retry soft bounces 3 times over 72 hours, then suppress
  4. Validate email addresses at signup — Use email verification APIs or double opt-in to prevent invalid addresses from entering your system
  5. Monitor sender reputation — Use tools like Google Postmaster Tools and your email provider's reputation dashboard to track your domain health

Real developers can help you.

Matthew Butler Matthew Butler Systems Development Engineer @ Amazon Web Services Nam Tran Nam Tran 10 years as fullstack developer Franck Plazanet Franck Plazanet I am a Strategic Engineering Leader with over 8 years of experience building high-availability enterprise systems and scaling high-performing technical teams. My focus is on bridging the gap between complex technology and business growth. Core Expertise: 🚀 Leadership: Managing and coaching teams of 15+ engineers, fostering a culture of accountability and continuous improvement. 🏗️ Architecture: Enterprise Core Systems, Multi-system Integration (ERP/API/ETL), and Core Database Structure. ☁️ Cloud & Scale: AWS Expert; architected systems handling 10B+ monthly requests and managing 100k+ SKUs. 📈 Business Impact: Aligning tech strategy with P&L goals to drive $70k+ in monthly recurring revenue. I thrive on "out-of-the-box" thinking to solve complex technical bottlenecks and am always looking for ways to use automation to improve business productivity. Daniel Vázquez Daniel Vázquez Software Engineer with over 10 years of experience on Startups, Government, big tech industry & consulting. zipking zipking I am a technologist and product builder dedicated to creating high-impact solutions at the intersection of AI and specialized markets. Currently, I am focused on PropScan (EstateGuard), an AI-driven SaaS platform tailored for the Japanese real estate industry, and exploring the potential of Archify. As an INFJ-T, I approach development with a "systems-thinking" mindset—balancing technical precision with a deep understanding of user needs. I particularly enjoy the challenge of architecting Vertical AI SaaS and optimizing Small Language Models (SLMs) to solve specific, real-world business problems. Whether I'm in a CTO-level leadership role or hands-on with the code, I thrive on building tools that turn complex data into actionable value. Meïr Ankri Meïr Ankri Full-stack developer specializing in React / Next.js / Node.js with 6+ years of experience. I've worked across various sectors including automotive (Reezocar/Société Générale), healthcare (Medical Link SaaS), and e-commerce (Glasman). I build web apps end-to-end, from architecture to production, with a focus on scalability, performance, and code quality. I also mentor junior developers and contribute to technical decisions and code reviews. MFox MFox Full-stack professional senior engineer (15+years). Extensive experience in software development, qa, and IP networking. Jared Hasson Jared Hasson Full time lead founding dev at a cyber security saas startup, with 10 yoe and a bachelor's in CS. Building & debugging software products is what I've spent my time on for forever David Olverson David Olverson Solo dev shipping production apps with AI-assisted development. I specialize in rescuing broken Lovable/Bolt/Cursor builds and taking them to production. 10+ apps shipped including SaaS CRMs, gaming platforms, real estate tools, and Discord bots. Stack: Next.js 16, TypeScript, Tailwind CSS, FastAPI, PostgreSQL, Prisma. I use Claude Code with 50+ custom skills for rapid delivery. Average turnaround: 2-4 weeks from broken prototype to production. Sage Fulcher Sage Fulcher Hey I'm Sage! Im a Boston area software engineer who grew up in South Florida. Ive worked at a ton of cool places like a telehealth kidney care startup that took part in a billion dollar merger (Cricket health/Interwell health), a boutique design agency where I got to work on a ton of exciting startups including a photography education app, a collegiate Esports league and more (Philosophie), a data analytics as a service startup in Cambridge (MA) as well as at Phillips and MIT Lincoln Lab where I designed and developed novel network security visualizations and analytics. I've been writing code and furiously devoted to using computers to make people’s lives easier for about 17 years. My degree is in making computers make pretty lights and sounds. Outside of work I love hip hop, the Celtics, professional wrestling, magic the gathering, photography, drumming, and guitars (both making and playing them)

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Frequently Asked Questions

What's the difference between a hard and soft bounce?

A hard bounce means the address is permanently invalid (user doesn't exist, domain doesn't exist). A soft bounce is temporary (mailbox full, server temporarily down). Stop sending to hard bounces immediately; retry soft bounces a few times before suppressing.

How do I recover a damaged sender reputation?

Stop sending to all bounced and unengaged addresses immediately. Clean your list to only active, confirmed recipients. Start sending low volumes and gradually increase. It typically takes 2-4 weeks for reputation to recover.

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