Demystifying SMTP Error Messages: Understanding Bounce Codes and Their Meanings

Quick Takeaways

• Updated: Feb 28, 2024.
• Email is part of our daily lives, whether for personal or professional use.
• Sending and receiving email is not always seamless. Sometimes messages fail to reach recipients.
• When delivery fails, SMTP error messages (bounce messages) explain why.

What is SMTP and a bounce message?

SMTP, or Simple Mail Transfer Protocol, is the standard mail server to send and receive emails.

When an email hits a problem during transmission, the receiving server generates an SMTP error message. That message is also called a bounce message. It is sent back to the sender. Bounce messages contain information about why delivery failed.

Soft bounces vs hard bounces

• A soft bounce is a temporary issue. Examples include a full mailbox or a server timeout.
• A hard bounce is a permanent failure. Examples include an invalid address or a blocked domain.

Remove hard-bounced addresses from mailing lists to help maintain deliverability.

Common bounce types

General bounce

A general bounce occurs when the reason for failure is unclear or could be caused by various technical issues. It is typically treated as a soft bounce.

DNS issues

This bounce happens when the recipient's server has DNS-related problems. The server is temporarily unable to accept email. It is usually a soft bounce.

Mailbox full

If the recipient's mailbox is full, the sender receives a mailbox full bounce. It is initially a soft bounce. Repeated occurrences may escalate to a hard bounce.

Message too large

When an email exceeds the recipient's mailbox size limits, the server returns a message size too large bounce.

Relaying error

Relaying errors occur when a temporary error prevents relaying the email. Causes include authentication issues or misconfigured servers. Fixing the root cause is crucial.

Spam-related blocks

If the recipient's server determines the email looks like spam, it may block delivery. This is usually treated as a soft bounce. Review email content and sender reputation to prevent future blocks.

Transient bounce

In a transient bounce, the recipient's server cannot deliver the email immediately but will retry for a limited time. It is treated as a soft bounce.

Challenge or response bounces

Some recipients use anti-spam measures that require senders to take an action before delivery. These challenge or response bounces require sender interaction.

Hard bounce

A hard bounce indicates a permanent delivery failure. Causes include an invalid email address, inactive mailbox, or blocked domain.

Error codes overview

SMTP error messages use numeric codes. These codes give extra detail about the delivery failure. They come in traditional and enhanced formats. Traditional codes include numbers like 421 (temporary failure) and 550 (permanent failure). Enhanced codes include numbers like 5.1.0 or 5.7.1 and provide more specific detail.

Email Error Codes In Depth Explained

Code — Type — Explanation

• 420 — Soft — Timeout connection problem: there were issues during the message transfer.
• 421 — Soft — The recipient server temporarily deferred the message due to too many messages or too many connections in a short timeframe.
• 422 — Soft — The recipient’s mailbox has exceeded its storage limit.
• 431 — Soft — Not enough space on the disk or an ‘out of memory’ condition due to a file overload.
• 432 — Soft — Typical side-message: ‘The recipient’s exchange server’s incoming mail queue has been stopped’.
• 441 — Soft — The recipient’s server is not responding.
• 442 — Soft — The connection dropped during the transmission.
• 446 — Soft — The maximum hop count has exceeded for the message: an internal loop has occurred.
• 447 — Soft — Your outgoing message timed out because of issues concerning the incoming server.
• 449 — Soft — A routing error - bad routing configuration or temporary routing issues, users may encounter an exchange server error.
• 450 — Soft — Requested mail action not taken: mailbox unavailable. (For example, mailbox busy).
• 451 — Soft — Requested action aborted: error in processing.
• 452 — Soft — Requested action not taken: insufficient system storage.
• 471 — Soft — An error in your mail server, often due to an issue with the local anti-spam filter.
• 500 — Hard — The server could not recognize the command due to a syntax error.
• 501 — Hard — A syntax error was encountered in command arguments.
• 502 — Hard — This command is not implemented.
• 503 — Hard — The server has encountered a bad sequence of commands.
• 504 — Hard — The command parameter is not implemented.
• 510 — Hard — Bad email address.
• 511 — Hard — Bad email address.
• 512 — Hard — A DNS error: the host server for the recipient’s domain name cannot be found.
• 513 — Hard — ‘Address type is incorrect’: another problem concerning address misspelling. In few cases, however, it’s related to an authentication issue.
• 523 — Hard — The total size of your mailing exceeds the recipient server’s limits.
• 530 — Hard — Typically, an authentication problem, but sometimes, it is about the recipient’s server blacklisting yours or an invalid email address.
• 541 — Hard — The recipient address rejected your message: typically, it is an error caused by an anti-spam filter.
• 550 — Hard — The user’s mailbox was unavailable (such as - not found).
• 551 — Hard — The recipient is not local to the server.
• 552 — Hard — The action was aborted due to exceeded storage allocation.
• 553 — Hard — The command was aborted because the mailbox name is invalid.
• 554 — Hard — The transaction failed for some unstated reason.
• 4.2.2 — Soft — The email account is over quota.
• 5.0.0 — Hard — The address does not exist.
• 5.1.0 — Hard — Another address status.
• 5.1.1 — Hard — A bad destination mailbox address.
• 5.1.2 — Hard — A bad destination system address.
• 5.1.3 — Hard — Bad destination mailbox address syntax.
• 5.1.4 — Hard — Destination mailbox address ambiguous.
• 5.1.5 — Hard — Destination mailbox address valid.
• 5.1.6 — Hard — Mailbox has moved.
• 5.1.7 — Hard — Bad sender’s mailbox address syntax.
• 5.1.8 — Hard — Bad sender’s system address.
• 5.2.0 — Soft — Other or undefined mailbox status.
• 5.2.1 — Soft — Mailbox disabled, not accepting messages.
• 5.2.2 — Soft — Mailbox full.
• 5.2.3 — Hard — Message length exceeds the administrative limit.
• 5.2.4 — Hard — Mailing list expansion problem.
• 5.3.0 — Hard — Other or undefined mail system status.
• 5.3.1 — Soft — Mail system full.
• 5.3.2 — Hard — The system is not accepting network messages.
• 5.3.3 — Hard — The system is not capable of selected features.
• 5.3.4 — Hard — Message too big for the system.
• 5.4.0 — Hard — Other or undefined network or routing status.
• 5.4.1 — Hard — No answer from the host.
• 5.4.2 — Hard — Bad connection.
• 5.4.3 — Hard — Routing server failure.
• 5.4.4 — Hard — Unable to route.
• 5.4.5 — Soft — Network congestion.
• 5.4.6 — Hard — Routing loop detected.
• 5.4.7 — Hard — Delivery time expired.
• 5.5.0 — Hard — Other or undefined protocol status.
• 5.5.1 — Hard — Invalid command.
• 5.5.2 — Hard — Syntax error.
• 5.5.3 — Soft — Too many recipients.
• 5.5.4 — Hard — Invalid command arguments.
• 5.5.5 — Hard — Wrong protocol version.
• 5.6.0 — Hard — Other or undefined media error.
• 5.6.1 — Hard — Media not supported.
• 5.6.2 — Hard — The conversion required and prohibited.
• 5.6.3 — Hard — A conversion required but not supported.
• 5.6.4 — Hard — A conversion with loss performed.
• 5.6.5 — Hard — Conversion failed.
• 5.7.0 — Hard — Other or undefined security status.
• 5.7.1 — Hard — Delivery not authorized, message refused.
• 5.7.2 — Hard — Mailing list expansion prohibited.
• 5.7.3 — Hard — Security conversion required but not possible.
• 5.7.4 — Hard — Security features not supported.
• 5.7.5 — Hard — Cryptographic failure.
• 5.7.6 — Hard — The cryptographic algorithm not supported.
• 5.7.7 — Hard — Message integrity failure.
• 9.1.1 — Hard — Hard bounce with no bounce code found (invalid email or rejected email from your mail server such as - sending limit).
• 5.1.10 — Hard — RESOLVER.ADR.RecipientNotFound; Recipientrebaz.jalizada@se.mcd.comnot found by SMTP address lookup.
• 5.4.316 — Soft — Message expired, connection refused.
• 4.2.1 — Hard — mailbox temporarily disabled.
• 4.4.2 — Hard — The email account that you tried to reach does not exist.

Download The Full List

Tags: