Is Vade's 550 5.7.1 always a hard bounce?

While `550 5.7.1` broadly indicates a security or policy violation, Vade often uses it with internal sub-codes (like `OXSUS001_403`). If this sub-code signifies a "non-existent customer/user," then it should be treated as a hard bounce, requiring immediate removal of the address from your list.

How are SMTP error codes structured?

SMTP error codes consist of three digits (e.g., `550`) and an optional enhanced status code (e.g., `5.7.1`). The first digit indicates the general status (e.g., `5xx` for permanent failure), and subsequent digits provide more specific details about the issue.

What is the difference between a hard and soft bounce?

A hard bounce indicates a permanent failure (e.g., invalid address), requiring removal from your list. A soft bounce signifies a temporary issue (e.g., full inbox), where the sending server might retry delivery.

What happens if I don't manage hard bounces?

Failing to remove hard-bounced addresses leads to high bounce rates, which negatively impact your sender reputation. This can result in your emails being sent to spam, or your IP/domain being placed on a blacklist (or blocklist).

Is Vade's use of 550 5.7.1 a hard bounce, and what do the error codes mean? - Troubleshooting - Email deliverability - Knowledge base

Email bounce messages, while often cryptic, hold vital clues about your email deliverability. A common error, especially for those sending to domains protected by Vade, is the `550 5.7.1` response. This code can be particularly confusing because its generic nature sometimes masks the true reason for non-delivery.

Understanding whether such a bounce is a hard or a soft bounce, and what the specific sub-codes signify, is crucial for maintaining a healthy sender reputation and ensuring your messages reach their intended recipients. Let's delve into what this `550 5.7.1` from Vade typically means and how to interpret these critical error messages.

Understanding SMTP error codes

SMTP error codes are standardized responses from receiving mail servers that inform the sending server about the delivery status of an email. These codes are structured, with the first digit indicating the command status, the second indicating the category, and the third indicating more detail. A `5xx` code, like `550`, signifies a permanent negative completion reply, meaning the email was not delivered and the sending server should not try again without modifications.

The `5.7.1` enhanced status code specifically points to a security status, often indicating a general policy failure, such as the sender being blocked or the message violating a policy. According to the IANA SMTP enhanced status codes, this code typically means "Delivery not authorized, message refused." This broad category covers various issues from IP blocklistings to content rejections.

It's crucial to analyze the entire bounce message, not just the `550 5.7.1` portion, as mail servers often provide additional text to clarify the specific reason for the rejection. This extra information is key to troubleshooting, as the same numerical code can have different underlying causes across various email providers.

Vade's specific use of 550 5.7.1

When Vade (or any email security provider) returns a `550 5.7.1` error, it signifies that they are refusing the connection or message due to a policy or security reason. In the specific case mentioned, the message includes `OXSUS001_403`. This `_403` part is an internal Vade code, which, despite the `550 5.7.1` primary code, can provide more specific context about why the email was rejected.

From experience, Vade sometimes uses the `550 5.7.1` code broadly for various rejections, including when a user is non-existent. If Vade's documentation (or the full bounce message) indicates that `OXSUS001_403` means the message was sent to a "non-existent customer/user," then despite the general `5.7.1` security classification, this specific instance should be treated as a hard bounce.

This situation highlights a common challenge in email deliverability, where different email providers interpret and return error codes in slightly non-standard ways. While the `550 5.7.1` from Microsoft Exchange Online, for example, typically means access denied or security policy violation, Vade's internal sub-code provides the more precise "user unknown" context, making it a permanent failure that warrants removal of the address from your list.

Hard versus soft bounces explained

In the world of email deliverability, distinguishing between hard and soft bounces is fundamental. A hard bounce indicates a permanent delivery failure, meaning the email address is invalid, non-existent, or the domain name is incorrect. These addresses should be immediately removed from your mailing lists to protect your sender reputation.

Conversely, a soft bounce signals a temporary delivery issue. This could be due to a full inbox, a server being temporarily down, or the message size being too large. With soft bounces, the sending server typically retries delivery for a period. If the issue persists, the soft bounce may eventually convert into a hard bounce.

For the `550 5.7.1 Connection refused` error from Vade, especially when it points to a non-existent user (via the `_403` sub-code), this is unequivocally a hard bounce. Continuing to send to such an address can seriously damage your sender reputation, leading to more widespread blocklisting (or blacklisting) issues.

Hard bounce

Permanent failure: The email can never be delivered to that address.
Common causes: Invalid recipient, domain non-existent, recipient blocked.
Action required: Immediately remove the address from your mailing list.
Example codes:550 5.1.1 (user unknown), 550 5.7.1 (policy violation for non-existent user).

Impact on deliverability and sender reputation

Ignoring hard bounces, especially those indicating non-existent users, is one of the quickest ways to damage your sender reputation. Email service providers (ESPs) and mailbox providers closely monitor bounce rates. High bounce rates signal to them that you might be sending to old, invalid, or purchased lists, which are indicators of potential spamming behavior. This can lead to your IP addresses or domains being placed on a blocklist, causing your legitimate emails to be rejected or routed to the spam folder.

To mitigate this, always validate your email lists regularly and promptly remove hard-bounced addresses. Ensuring your email authentication protocols like DMARC, SPF, and DKIM are correctly configured also plays a significant role. These protocols verify your sending identity, reducing the likelihood of your emails being flagged as suspicious or spam, even if they encounter a temporary issue.

A robust bounce management strategy is part of a broader email deliverability effort. By accurately parsing and acting upon bounce messages, you can maintain a clean mailing list, build trust with mailbox providers, and ultimately achieve better inbox placement. Remember, every bounce message, even the ambiguous ones, offers an opportunity to refine your sending practices.

Code	Classification	Common meaning
500	Syntax error	Command not recognized or syntax error in command.
550	Mailbox unavailable/not found	Recipient's mailbox is invalid, non-existent, or rejected for policy reasons (often a hard bounce).
551	User not local	Recipient is not on the server to which the email was sent, or relaying denied.
552	Storage allocation exceeded	Recipient's mailbox has exceeded its storage limit. Usually a soft bounce.
554	Transaction failed	General permanent failure, often due to spam filters or policy violations.

Views from the trenches

Best practices

Always interpret any indication of a non-existent user as a hard bounce, regardless of the primary SMTP code, and remove the address.

Regularly clean your email lists to remove invalid or inactive addresses to prevent high bounce rates.

Implement and monitor DMARC, SPF, and DKIM to strengthen your email authentication and sender reputation.

Review full bounce messages, including sub-codes and explanatory text, for precise diagnostic information.

Common pitfalls

Treating a 'user unknown' bounce as a soft bounce, leading to continued sending and reputation damage.

Ignoring the specific internal codes provided by services like Vade, which clarify generic SMTP errors.

Failing to regularly validate email addresses, resulting in higher bounce rates over time.

Relying solely on the main SMTP code without analyzing the full context of the bounce message.

Expert tips

Some providers may use a `550 5.7.1` for various rejections, including invalid users.

The `_403` in Vade's message is an internal code; it usually indicates a permanent failure.

ESP's can sometimes misinterpret bounce codes, so understanding the RFCs is important.

A `550 5.1.1` should be used for unknown users, but some providers use `550 5.7.1` instead.

Expert view

Expert from Email Geeks says that many providers set the `550 5.7.1` as a hard bounce based on their internal documentation, signifying a permanent refusal.

2022-03-29 - Email Geeks

Expert view

Expert from Email Geeks says that the `403` is not an SMTP error code and that the first three digits in an error message constitute the actual error code, as defined in RFC2821.

2022-03-30 - Email Geeks

Moving forward with bounce management

The `550 5.7.1` bounce from Vade, when accompanied by sub-codes like `OXSUS001_403` indicating a non-existent user, should be treated as a hard bounce. This means the email address is permanently unreachable and must be removed from your mailing list to protect your sender reputation.

Understanding the nuances of SMTP error codes and the specific contexts provided by various mail services is key to effective email deliverability. By diligently managing bounces and keeping your lists clean, you ensure your emails reach valid recipients, improve your inbox placement, and avoid getting on an email blocklist (or blacklist).