📩 Understanding Email Bouncebacks: Common Error Messages & Solutions 🚀 Print

✨ Introduction

An email bounceback occurs when an email fails to reach its recipient, triggering a return notification to the sender. This typically happens due to incorrect email settings, server-side issues, DNS errors, or spam filtering policies.

Emails can bounce back for different reasons, which are categorized into two primary types:

✅ Deferrals (400-series errors): Temporary issues where the mail server will attempt to resend the message.
✅ Fatal Bounces (500-series errors): Permanent failures where the email cannot be delivered and requires corrective action.

This guide will help you understand common SMTP bounce codes, diagnose the cause, and take action to resolve the issues efficiently. ✅

🔍 1. Connection & Server-Related Errors 🌐

These errors indicate connectivity issues between mail servers.

• 101 — The server is unable to connect (incorrect server name or port issue). ❌

• 111 — Connection refused (SMTP server is unreachable due to firewall restrictions or misspelled domains). 🔥

• 211 — System status message (informational response). ℹ️

• 214 — HELP command (server FAQ response). 📝

📤 2. Transmission & Delivery Errors 🚚

These errors indicate issues related to mail routing and processing.

• 220 — Server is ready. ✅

• 221 — Server is closing transmission. 🔄

• 250 — Mail successfully delivered. ✅

• 251 — User not local; forwarding to a different server. 🔁

• 252 — Cannot verify recipient; will attempt redelivery. ⏳

• 253 — Pending messages for server. 📨

📬 3. Mail Input & Routing Errors 📍

Errors related to incorrect mail routing, network failures, or delivery timeouts.

• 354 — Start mail input (Response to DATA command). 🖊️

• 420 — Timeout connection issue (firewall or hardware-related). ⏳

• 421 — Exceeded simultaneous connections; service unavailable. ❌ 🔗 Fix Exim 421 Errors

• 422 — Recipient’s mailbox is full. 📦

• 431 — Server out of memory. 🛑

• 441 — Recipient's mail server is not responding. ⚠️

• 450 — Mailbox unavailable (server is down, blacklisted, or IP blocked). 🚫

• 451 — Local processing error (spam filtering or bulk email rejection). 🔥

• 452 — Too many recipients or emails sent in a short time. 📈

• 454 — TLS not available; authentication required. 🔐

🔑 4. Syntax, Authentication & Mailbox Errors 🔄

Errors related to incorrect commands, authentication failures, or mailbox restrictions.

• 500 — Syntax error (command not recognized). ❌

• 501 — Invalid email address format. 📭

• 502 — Command not implemented. 🔄

• 503 — Bad command sequence (authentication required). ⚠️

• 504 — Command parameter not implemented. 🛠️

• 510/511 — Invalid recipient address. 📛

• 512 — DNS error; domain cannot be found. 🌐

• 515 — Invalid destination mailbox address. 📭

• 521 — Domain does not accept incoming mail. 🚫

• 522 — Recipient’s mailbox storage exceeded. 📦

• 523 — Message too large for server. 📏

• 530 — Authentication required for email sending. 🔑

• 535 — Authentication failed (wrong username/password). 🚫

• 550 — Mailbox unavailable (non-existent or blocked). 🚫

• 551 — User not local; please forward to another address. 🔁

• 552 — Storage allocation exceeded; mailbox is full. 📦

• 553 — Invalid recipient mailbox format. ❌

• 554 — Transaction failed; message flagged as spam or sender IP is blacklisted. 🛑

🛠 5. How to Fix Email Bouncebacks? ✅

To resolve bounceback issues, follow these troubleshooting steps:

✅ Verify recipient’s email address: Ensure the email is correct and active.
✅ Check DNS records: Confirm that MX, SPF, and DKIM records are correctly configured.
✅ Monitor email sending limits: Reduce bulk emails or request an increase in limits.
✅ Authenticate email properly: Use SMTP authentication and secure connection settings.
✅ Check for blacklisting: Use MxToolbox to verify if your domain/IP is blacklisted.
✅ Review server logs: Check logs like /var/log/exim_mainlog to identify specific failures.
✅ Contact your web host: If the issue persists, reach out to Domain India Support.

📌 Need More Help? 🔹 Managing Exim Mail Queue: Comprehensive Guide
🔹 Troubleshooting SMTP Relay Issues in VPS Hosting 

🔹Diagnosing Email Delivery Failures: A Case Study and Additional Insights

🚀 Stay Ahead in Email Management! Understanding these error messages will help you diagnose and fix email delivery problems quickly, ensuring smooth communication. ✅