Summary
To maintain optimal email deliverability and sender reputation, marketers should primarily suppress email addresses that generate SMTP 5xx bounce codes. These codes, such as 550, 551, 553, 554, and 555, signify permanent delivery failures, commonly known as hard bounces. Such failures occur for reasons like non-existent mailboxes, invalid recipient addresses, recipient rejections, or policy blocks. While soft bounces indicate temporary issues, an email address consistently generating soft bounces across multiple campaigns may eventually be converted to a hard bounce by an ESP and subsequently suppressed. Immediate suppression of hard-bounced addresses is vital to prevent ongoing harm to a sender's reputation, even though ESPs may classify bounce types differently, and some less common 5xx errors could technically be temporary.
Key findings
- 5xx Codes for Suppression: SMTP bounce codes in the 5xx range are the primary indicators for permanent delivery failures, often referred to as hard bounces, and necessitate immediate mailing list suppression.
- Common 5xx Examples: Specific 5xx codes frequently cited for suppression include 550 (mailbox not found, invalid recipient, policy rejection), 551 (user not local), 553 (invalid address), 554 (transaction failed, often due to spam blocks or invalid recipient), and 555. These codes confirm that an email address is invalid, non-existent, or permanently unreachable.
- Protecting Sender Reputation: Immediately suppressing email addresses that generate 5xx bounce codes is crucial for protecting sender reputation and maintaining high deliverability. Continuing to send to these undeliverable addresses signals poor list hygiene to Internet Service Providers (ISPs), which can negatively impact future email campaigns.
Key considerations
- Varying ESP Classifications: Email Service Providers (ESPs) may classify bounce types, such as hard, soft, or spam, differently, even for the same underlying SMTP error. This means an error like 'DNS Failure' might be a soft bounce with one ESP, but handled differently by another.
- Soft Bounce Conversion: While soft bounces indicate temporary issues, persistent soft bounces for an email address across multiple campaigns can lead to its conversion into a hard bounce by an ESP, ultimately resulting in suppression. For instance, some ESPs, like Mailchimp, convert an address to a hard bounce after 7 to 15 repeated soft bounces.
- Exceptions to 5xx Rules: Although 5xx codes generally denote permanent failures, some rare cases, like a 552 'mailbox full' error, can technically be temporary. However, most experts still recommend suppressing these addresses due to their high likelihood of recurrence and their impact on sender reputation.
What email marketers say
11 marketer opinions
Marketers should prioritize suppressing email addresses that trigger SMTP 5xx bounce codes, as these universally signal permanent delivery failures, often termed hard bounces. Common examples like 550, 551, 553, and 554 indicate issues such as non-existent mailboxes, invalid recipients, or outright rejections. Removing these addresses promptly is critical for preserving sender reputation and overall deliverability. While temporary soft bounces do not warrant immediate suppression, a consistent pattern of soft bounces for a particular address across multiple campaigns can lead to its conversion into a hard bounce by many email service providers, eventually requiring its removal from the mailing list.
Key opinions
- Immediate Suppression for 5xx Bounces: Email addresses generating SMTP 5xx bounce codes, which denote permanent delivery failures or hard bounces, require immediate suppression from mailing lists. This includes common codes like 550, 553, and 554, which signify non-existent mailboxes, invalid addresses, or transactional failures.
- Permanent Failure Indicators: Codes such as 550 (mailbox not found, invalid recipient), 551 (user not local), and 554 (transaction failed, often due to spam blocks) are consistently identified as hard bounces, meaning the email address is permanently unreachable or invalid.
- Reputation Preservation: Swiftly removing hard-bounced email addresses is essential for protecting a sender's reputation, as continued attempts to send to invalid or non-existent recipients negatively impacts deliverability signals to Internet Service Providers (ISPs).
Key considerations
- Soft Bounce Thresholds: While immediate suppression is reserved for hard bounces, temporary soft bounces warrant monitoring. Email Service Providers like Mailchimp may convert an email address to a hard bounce, requiring suppression, if it repeatedly soft bounces across campaigns, often after 7 to 15 attempts.
- Universal 5xx Application: The consensus among email marketing experts is that any SMTP code in the 5xx range indicates a permanent delivery failure. These codes, such as 550, 551, 553, and 554, consistently signal issues like non-existent mailboxes or recipient rejections, demanding immediate list suppression.
- Proactive Reputation Management: The primary driver for immediate suppression of 5xx hard bounces is to prevent severe damage to sender reputation. Continuously attempting delivery to invalid addresses signals poor list hygiene to Internet Service Providers, negatively impacting future campaign deliverability.
Marketer view
Marketer from Email Geeks explains that 5xx errors are generally unrecoverable, suggesting immediate removal from the list. He further details that soft bounces are temporary errors, but if an email address continues to soft bounce across multiple campaigns, it may eventually be converted to a hard bounce and cleaned from the audience, citing Mailchimp's policy of allowing seven to fifteen soft bounces before conversion.
17 Jan 2022 - Email Geeks
Marketer view
Email marketer from Mailgun explains that SMTP codes in the 5xx range, such as 550, 553, and 554, indicate a permanent failure and are classified as hard bounces, meaning the address is invalid or non-existent and should lead to immediate mailing list suppression.
24 Jul 2024 - Mailgun
What the experts say
3 expert opinions
Email marketers should prioritize suppressing email addresses that generate SMTP 5xx bounce codes, as these are widely recognized as permanent delivery failures or hard bounces. Common examples such as 550, 551, 553, 554, and 555 typically indicate non-existent mailboxes, invalid recipient addresses, or policy rejections, necessitating immediate removal from mailing lists. While 5xx codes generally denote permanent issues, it is important to recognize that Email Service Providers, or ESPs, apply their own classifications, which can vary. This means that an error like 'DNS Failure' might be categorized as a soft bounce by some ESPs. Furthermore, even a 552 'mailbox full' code, despite its potential for being temporary, is often recommended for suppression due to the high likelihood of recurrence and its impact on sender reputation. Swift suppression of these permanent failures is crucial for maintaining a strong sender reputation and ensuring optimal deliverability.
Key opinions
- Immediate 5xx Suppression: SMTP bounce codes in the 5xx range, which include common examples like 550, 551, 553, 554, and 555, are strong indicators of permanent delivery failures and warrant immediate mailing list suppression.
- Permanent Failure Types: These 5xx codes consistently signify issues such as non-existent mailboxes, invalid recipient addresses, policy-based rejections, or other fatal errors that prevent email delivery permanently.
- Protecting Sender Reputation: Promptly suppressing email addresses associated with 5xx bounce codes is essential for safeguarding sender reputation. Continuing to send to these undeliverable addresses signals poor list hygiene to Internet Service Providers, negatively impacting overall deliverability.
Key considerations
- Varying ESP Classifications: Email Service Providers, or ESPs, classify bounce types - hard, soft, or spam - with their own unique systems, and these classifications can differ significantly between providers. For instance, a 'DNS Failure' might be deemed a soft bounce by one ESP, while another might categorize it differently.
- 552 Code Suppression: While most 5xx SMTP bounce codes unequivocally signal permanent failures, the 552 'mailbox full' code, though potentially a temporary issue, should generally lead to suppression. Experts recommend this due to its high likelihood of recurrence and the negative impact on sender reputation from repeatedly sending to full inboxes.
- Beyond Standard 5xx: Although 5xx codes are the primary focus for suppression, the ultimate decision on which bounce codes lead to permanent removal can be influenced by an ESP's specific internal logic and how they interpret and categorize different SMTP responses.
Expert view
Expert from Email Geeks explains that the classifications of hard, soft, and spam bounces are assigned by Email Service Providers (ESPs), and these classifications can differ between various ESPs. She clarifies that even errors like 'DNS Failure' are often categorized as soft bounces by ESPs, highlighting the variability in how different providers handle bounce classifications.
8 Oct 2021 - Email Geeks
Expert view
Expert from Spam Resource explains that SMTP 5xx bounce codes generally indicate permanent failures, also known as hard bounces, and should lead to immediate mailing list suppression. Specific examples often include 550, 551, 553, 554, and 555, which typically signify issues like non-existent mailboxes, policy rejections, or other fatal errors.
30 Nov 2024 - Spam Resource
What the documentation says
4 technical articles
SMTP bounce codes in the 5xx range consistently indicate permanent delivery failures and are the definitive signal for mailing list suppression. This universal recommendation spans official standards, such as RFC 5321, and is echoed by major email service providers and mail server software, including SendGrid, Microsoft Exchange, and Postfix. These permanent failures, often referred to as hard bounces, arise from issues like non-existent mailboxes, invalid recipient addresses, or outright recipient rejections, making the email undeliverable. Suppressing these addresses promptly is critical for safeguarding sender reputation, preventing wasted sending efforts, and ensuring optimal email deliverability.
Key findings
- Universal Permanent Failure: SMTP reply codes in the 5xx range universally indicate permanent negative completion replies, meaning the command failed and the recipient cannot be reached, making them the fundamental type of error for mailing list suppression, as outlined in RFC 5321.
- Examples from Major Providers: Leading email service providers like SendGrid confirm that 5xx status codes, such as 550 for 'mailbox not found' or 'invalid domain,' signify a hard bounce requiring immediate suppression. Similarly, Microsoft Learn's documentation on DSNs in Exchange points to 5.x.x codes like 'bad destination mailbox address' (5.1.1) as clear indicators for suppression.
- Mail Server Behavior: Mail transfer agents like Postfix also treat 5xx SMTP reply codes as permanent delivery errors, such as 'unknown user.' While Postfix ceases retrying these, the implication for email senders is to permanently suppress such addresses to maintain sender reputation and avoid future delivery failures.
Key considerations
- Sender Reputation Protection: The primary objective of suppressing addresses generating 5xx bounce codes is to actively protect and maintain a strong sender reputation. Continuously attempting to send emails to permanently invalid addresses signals poor list hygiene to Internet Service Providers, or ISPs, leading to deliverability issues for legitimate mail.
- Consistency Across Systems: The consensus for suppressing 5xx codes is reinforced by the consistent behavior of mail server software, such as Postfix, which automatically ceases retry attempts for these permanent errors. This underscores the need for senders to follow suit with suppression.
- Proactive List Management: Acting on 5xx bounce codes enables proactive list management, preventing future email sending attempts to non-existent or unreachable recipients. This not only improves deliverability but also optimizes sending resources by focusing on valid, engaged subscribers.
Technical article
Documentation from RFC 5321 explains that 5xx SMTP reply codes indicate permanent negative completion replies, meaning the command failed and the recipient cannot be reached, which are the fundamental types of errors that should lead to mailing list suppression.
18 Feb 2025 - RFC 5321
Technical article
Documentation from SendGrid states that 5xx status codes, such as 550 - mailbox not found, invalid domain, or recipient rejected - indicate a permanent failure. These hard bounces signify an undeliverable email and require the recipient's address to be suppressed from future mailings.
6 Mar 2022 - SendGrid
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