Where can I find a comprehensive list of bounce codes?

While there isn't one master list, the most authoritative resources include the IANA official SMTP enhanced status codes and practical guides like the SMTP Field Manual . Additionally, your email service provider's documentation and major mailbox providers' postmaster pages often provide specific explanations for the codes you encounter.

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

A hard bounce indicates a permanent delivery failure, like an invalid email address, and requires immediate removal of the address from your list. A soft bounce is a temporary issue, such as a full mailbox or a busy server, and ESPs typically retry delivery. If soft bounces persist, they can eventually be treated as hard bounces.

How do I know if my emails are being blocked by a blacklist?

When your domain or IP is on an email blocklist or blacklist , mail servers will reject your emails, often with a 550 5.7.1 error message referencing the blocklist. This severely impacts your deliverability and requires you to address the underlying cause (e.g., spam complaints) and request delisting.

Is the numerical code enough, or do I need to read the full bounce message?

The full bounce message, including any human-readable text alongside the numerical SMTP code, provides critical context. While the code gives a general classification, the text often specifies the exact reason for the bounce, such as "mailbox not found" or "account suspended" . Always examine the full message for accurate diagnosis.

Where can I find a list of bounce/block codes and their explanations? - Troubleshooting - Email deliverability - Knowledge base

Email deliverability can feel like a labyrinth, especially when you encounter cryptic bounce and block codes. These messages, often in the form of SMTP (Simple Mail Transfer Protocol) status codes, are crucial feedback from recipient mail servers, telling you why your email couldn't be delivered. Understanding them is the first step to improving your email program and ensuring your messages reach the inbox.

The challenge is that there isn't one universal, exhaustive list of all bounce and block codes with precise explanations. While core SMTP standards provide a foundation, many mailbox providers and email service providers (ESPs) implement their own specific, sometimes vague, error messages and internal classifications. This means a code that signals a temporary issue on one server might indicate a permanent failure on another. It requires a strategic approach to interpret.

Understanding SMTP status codes

At its core, email delivery relies on SMTP, which uses numerical codes to communicate the status of an email transaction. These codes are typically three digits, like 250 for success or 550 for a permanent failure. Beyond these basic codes, there are enhanced status codes, which provide more granular detail, often in the format of X.Y.Z, such as 5.1.1 for an unknown user.

Generally, these codes fall into a few key categories. Codes starting with 2xx indicate success. Codes starting with 4xx signify a temporary failure, often referred to as a soft bounce, where the mail server will retry delivery later. Lastly, 5xx codes denote a permanent failure or hard bounce, meaning the email will not be delivered and retries are futile.

You can find the official definitions of enhanced SMTP status codes from the Internet Assigned Numbers Authority (IANA). However, it's worth noting that these are general guidelines, and real-world implementations by various email providers can introduce unique nuances or more specific, proprietary messages. For a practical, real-world perspective on interpreting these codes, resources like the SMTP Field Manual provide extensive examples of actual bounce responses from different ISPs and spam filters.

Example SMTP bounce messagestext

550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in local recipient table
550 5.7.1 Service unavailable; Client host [X.X.X.X] blocked using spam.blacklist.com; See: http://www.spam.blacklist.com/lookup.cgi?ip=X.X.X.X
451 4.7.1 Service unavailable - try again later

These messages often contain a combination of the numerical SMTP code and a human-readable explanation, which can range from very clear to quite vague. The actual text following the code provides context and is often more useful than the code itself for diagnosing the specific problem. It's important to differentiate between temporary issues and those that require immediate action, like removing an address from your mailing list.

Types of bounces and block codes

Bounce codes broadly classify delivery failures, but within these, we distinguish between soft and hard bounces, as well as distinct block codes. Knowing the difference is key to maintaining a healthy sender reputation and effective email campaigns. Your email service provider (ESP) should classify these bounces, but understanding the underlying codes helps you confirm their actions.

Hard bounce

A hard bounce indicates a permanent delivery failure. This means the email address is invalid, nonexistent, or the recipient's server has permanently blocked your messages. Continuing to send to these addresses will harm your sender reputation.

Common codes: Often 550 (mailbox not found), 553 (mailbox name invalid).
Action required: Immediately remove these addresses from your mailing list (suppress them). Repeated hard bounces can severely damage your sender reputation.

Soft bounce

A soft bounce signifies a temporary delivery issue. The recipient's server temporarily rejects the email, but it may be deliverable later. ESPs typically retry these emails for a period before classifying them as a hard bounce.

Common codes: Generally 4xx codes, such as 421 (service unavailable), 450 (mailbox busy), or 452 (insufficient storage).
Action required: Monitor and retry. If a soft bounce persists over time, it may convert to a hard bounce, requiring suppression. Different ESPs manage these differently.

Block codes (or blacklist codes) are a specific type of hard bounce. These occur when your sending IP address or domain is listed on a real-time blacklist (RBL) or blocklist by a major email security provider. This is a serious issue that demands immediate attention, as it can prevent all your emails from reaching a wide range of recipients.

Common block codes usually start with 550 5.7.1 or similar variations, often accompanied by a message stating "Service unavailable; Client host blocked" and a reference to the specific blacklist or blocklist that has listed your domain or IP. If you see this, you need to investigate the listing and request delisting.

Navigating code variations and resources

As previously mentioned, there isn't a single, definitive list that encompasses all possible bounce and block codes, as many mailbox providers (like

Gmail,

Outlook, and

Yahoo) and ESPs use their own internal or slightly modified codes and verbose descriptions. This makes comprehensive troubleshooting a dynamic process.

However, general categories and common examples are widely available. Your best starting points are often your own ESP's documentation, which usually explains the specific bounce codes they report. Additionally, postmaster pages from major mailbox providers can offer insights into their unique rejection reasons and blocklist policies. Here's a table of some common SMTP codes and their general meanings:

Code	Classification	Common Reason	Action Needed
421	Soft Bounce	Service not available (e.g., server too busy)	Retry later, ESP usually handles
450	Soft Bounce	Requested mail action not taken (e.g., mailbox busy)	Retry later, ESP usually handles
451	Soft Bounce	Local error in processing (e.g., temporary server issue)	Retry later
550	Hard Bounce	Mailbox not found, no such user, or access denied	Suppress address immediately
550 5.1.1	Hard Bounce	Invalid recipient or recipient unknown	Suppress address
550 5.7.1	Hard Bounce / Block	Blocked by recipient, blacklisted IP/domain, or general rejection	Investigate reputation issues, check blocklists
552	Hard Bounce	Quota exceeded or mailbox full	Treat as hard bounce, suppress (likely dormant)

While this table provides common codes, the accompanying text in the bounce message is often the most informative part. Always analyze the full bounce message to get the complete picture of why your email was rejected. This will help you decide whether it's a temporary hiccup or a more serious issue requiring immediate action.

Actioning your bounce data

Understanding bounce data goes beyond just identifying the codes, it's about using that information to refine your email strategy. First and foremost, any email address resulting in a hard bounce should be immediately suppressed from your mailing list. This prevents future sends to invalid addresses, which can significantly damage your sender reputation with mailbox providers.

For soft bounces, monitor their frequency. While ESPs often retry, a consistent soft bounce from a specific address might indicate a dormant account or a persistent server issue. After a certain number of retries or a predefined period, these should also be suppressed to keep your list clean and engaged. Regular list hygiene is critical for improving overall email deliverability.

Proactive steps to reduce bounces

Validate addresses: Implement a robust email validation process at the point of signup.
Monitor blocklists: Regularly check if your IP or domain is listed on any major email blocklists.
Segment inactive users: Isolate contacts who haven't engaged in a long time and consider removing them.
Authentication: Ensure your DMARC, SPF, and DKIM records are correctly configured.

By actively monitoring and responding to bounce and block codes, you prevent your email marketing efforts from being undermined by poor deliverability. This proactive approach not only keeps your lists clean but also helps build and maintain a strong sender reputation, ensuring your messages reliably reach their intended audience.

Views from the trenches

Best practices

Act quickly on hard bounces, suppressing those email addresses right away to protect your sender reputation.

Regularly monitor your ESP's bounce reports and any specific blocklist notifications.

Implement email validation at signup to prevent invalid addresses from entering your list.

Understand that context matters; the text accompanying an SMTP code often provides crucial details.

Common pitfalls

Ignoring soft bounces, which can eventually become hard bounces and indicate a deteriorating list.

Failing to check if your IP or domain is on any email blocklists or blacklists.

Not maintaining proper email authentication (SPF, DKIM, DMARC), which can lead to rejections.

Focusing solely on numerical codes without reading the full bounce message from the server.

Expert tips

Use an email deliverability monitoring tool to get real-time insights into bounce rates.

Segment your audience based on engagement to identify inactive subscribers who might soft bounce.

Consult mailbox provider postmaster pages for their specific guidelines and error explanations.

Automate suppression for hard bounces to ensure immediate removal and list cleanliness.

Expert view

Expert from Email Geeks says the SMTP field manual is a highly referenced resource for understanding bounce codes in the wild.

2021-11-04 - Email Geeks

Expert view

Expert from Email Geeks says that while IANA enumerates ESMTP status codes, many ISPs and filters use their own proprietary codes, which can make official documentation less useful in practice.

2021-11-04 - Email Geeks

Improving email deliverability

While a single, comprehensive list of every possible email bounce and block code remains elusive due to the varied implementations across different mail systems, understanding the general categories and common codes is immensely powerful. By distinguishing between soft and hard bounces, and recognizing when your sending reputation is affected by a blacklist or blocklist, you gain actionable insights.

The key is to proactively monitor your bounce data, understand the nuances of the full bounce messages, and implement robust list hygiene practices. This diligent approach ensures your emails land in the inbox, contributing to the success of your email campaigns and maintaining a healthy sender reputation over time.