Claude Code email

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.

MFox MFox Full-stack professional senior engineer (15+years). Extensive experience in software development, qa, and IP networking. Nam Tran Nam Tran 10 years as fullstack developer Vlad Temian Vlad Temian 15+ years shipping production infrastructure for startups. Former CTO at qed.builders (acquired by The Sandbox). Cursor ambassador and agentic tooling builder. I've scaled systems, automated deployments, and built observability tools for AI coding workflows. I specialize in taking vibe-coded apps from broken prototype to production-ready: fixing Supabase auth/RLS, Stripe integrations, deployment pipelines, and cleaning up AI-generated spaghetti. I build tools in this space (agentprobe, claudebin, micode) and understand both sides: how AI generates code and why it breaks. https://blog.vtemian.com/ Jen Jacobsen Jen Jacobsen I’m a Full-Stack Developer with over 10 years of experience building modern web and mobile applications. I enjoy working across the full product lifecycle — turning ideas into real, well-built products that are intuitive for users and scalable for businesses. I particularly enjoy building mobile apps, modern web platforms, and solving complex technical problems in a way that keeps systems clean, reliable, and easy to maintain. ISHANTDEEP SINGH ISHANTDEEP SINGH Senior Software Engineer with 7+ years of experience in React, JavaScript, TypeScript, Next.js, and Node.js. I’ve also worked as a tech lead for startups, owning end-to-end technical execution including architecture, development, scaling, and delivery. I bring a strong mix of hands-on coding, product thinking, and technical leadership, and I’m comfortable building products from scratch as well as improving and scaling existing systems. Krishna Sai Kuncha Krishna Sai Kuncha Experienced Professional Full stack Developer with 8+ years of experience across react, python, js, ts, golang and react-native. Developed inhouse websearch tooling for AI before websearch was solved : ) Costea Adrian Costea Adrian Embedded Engineer specilizing in perception systems. Latest project was a adas camera calibration system. Dor Yaloz Dor Yaloz SW engineer with 6+ years of experience, I worked with React/Node/Python did projects with React+Capacitor.js for ios Supabase expert BurnHavoc BurnHavoc Been around fixing other peoples code for 20 years. Jacek Rozanski Jacek Rozanski Senior PHP/Symfony developer and DevOps engineer with 20+ years of professional experience, running opcode.pl (web development agency, est. 2004). Day job: I'm the sole backend developer at merketing company where I own and maintain 11 PHP/Symfony microservices on AWS (ECS Fargate, RDS, S3, CloudFront), handle the full CI/CD pipeline (Bitbucket Pipelines, Docker), and manage monitoring with Sentry and CloudWatch. These services handle high request volumes in production every month. What I bring to AI-built apps: - I audit and fix security issues (OWASP methodology), performance bottlenecks, and architectural problems in codebases generated by Cursor, Claude Code, Lovable, Bolt, and v0 - I refactor AI-generated prototypes into production-grade applications with proper error handling, testing, and clean architecture (SOLID, DDD, hexagonal architecture) - I set up the infrastructure AI tools don't touch: AWS hosting, CI/CD pipelines, automated deployments, database optimization, monitoring, and alerting - I integrate external services: payment providers, email systems, partner APIs, SSO/auth Tech stack: PHP 8.x, Symfony, React, Next.js, PostgreSQL, MySQL, Docker, AWS (ECS, RDS, S3, SQS/SNS, CloudFront), Terraform, Supabase. I also use AI tools daily (Claude Code, Cursor) in my own workflow, so I understand both the strengths and the gaps in AI-generated code. Based in Poland (CET timezone). Available for async work and calls during EU/US business hours.

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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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