Summary
The '550 5.1.1 Recipient address rejected: User unknown' email bounce error consistently signals that the recipient's email address does not exist on the destination mail server. This is categorized as a hard bounce, indicating a permanent delivery failure. It is typically generated by the recipient's Mail Transfer Agent (MTA) because it cannot locate the mailbox. Common causes include typos, outdated mailing lists, inactive accounts, or instances where the intended user accounts have not yet been created. Senders should promptly remove these invalid addresses from their lists to maintain deliverability and sender reputation.
Key findings
- Non-Existent Address: The '550 5.1.1 Recipient address rejected: User unknown' error universally signifies that the recipient's email address does not exist on the destination mail server or within their domain. It is effectively a literal interpretation that the user account cannot be found.
- Permanent Failure (Hard Bounce): This error code represents a hard bounce, meaning the email cannot be delivered and indicates a permanent failure. The email will not reach the intended recipient via that address.
- Recipient MTA Origin: The bounce message originates from the recipient's Mail Transfer Agent (MTA). This indicates that the recipient's server itself cannot locate the specified mailbox, rather than an issue with the sender's system or content.
- Common Causes: Typical reasons for this error include recipients having left their organization, accounts being deleted or inactive, or simply an incorrectly typed email address. These are all variations of the core problem- the address does not exist.
Key considerations
- Typo Correction and List Hygiene: The primary cause is often a simple typo in the email address or an outdated mailing list. Senders should verify recipient addresses carefully and regularly clean their lists by removing non-existent or inactive accounts to maintain good deliverability.
- Recipient-Side Issues: While usually indicating an invalid address, less common scenarios include a recipient's spam filter caching old responses or the intended user accounts not yet being created on the recipient's email system. In such cases, direct verification with the address owner may be beneficial, though this is rare.
- Action on Hard Bounces: Since this is a hard bounce, indicating a permanent failure, the affected email address should be promptly removed from your mailing list. Continued attempts to send to non-existent addresses can negatively impact your sender reputation.
What email marketers say
11 marketer opinions
When an email returns with a '550 5.1.1 Recipient address rejected: User unknown' bounce error, it is a clear indication that the intended email address does not exist on the recipient's mail server. This classification as a hard bounce signifies a permanent delivery failure, meaning the message will never reach that specific address. This error message is typically generated by the recipient's own Mail Transfer Agent (MTA) as it cannot locate a corresponding mailbox. The most frequent causes include simple typing errors, obsolete entries in mailing lists, or accounts that have become inactive or been deleted. In rarer cases, it could also point to a delay in user account creation on the recipient's system.
Key opinions
- Direct Indication of Invalid Address: The '550 5.1.1' code explicitly states that the recipient's email address is non-existent, invalid, or simply unknown to the mail server it's being sent to.
- Unrecoverable Delivery Failure: Classified as a hard bounce, this error denotes a permanent, unresolvable issue with the recipient's address, meaning the email cannot be delivered at all.
- Server-Side Rejection: The error is a direct rejection from the recipient's Mail Transfer Agent (MTA), indicating that their system cannot find the specified mailbox, ruling out sender-side issues like content or reputation as the immediate cause.
- Root Causes Vary: While often due to user input errors like typos or outdated contact information, the error can also stem from accounts being abandoned, deleted, or, in specific scenarios, not yet provisioned on the recipient's system.
Key considerations
- Proactive List Cleaning: To minimize these bounces, regularly audit and clean email lists to remove stale, inactive, or invalid addresses, preventing repeated delivery attempts to non-existent users.
- Verify New Addresses: Implement processes to verify newly acquired email addresses, such as double opt-in or real-time validation tools, to catch typos and non-existent accounts early.
- Immediate Address Removal: Upon receiving a '550 5.1.1' bounce, the associated email address should be immediately suppressed or removed from your active mailing list, as continuing to send to it harms sender reputation.
- Investigate Rare Cases: Although rare, if a crucial recipient generates this error, it may be worth investigating if their email system has misconfigurations, like cached old data, or if their accounts have simply not been set up yet, as these are exceptions to the 'permanently gone' rule.
Marketer view
Marketer from Email Geeks explains that the '550 5.1.1 Recipient address rejected: User unknown' error typically means the user account doesn't exist and is unrelated to the sender. They also suggest verifying with the address owner and considering if the recipient's spam filter service might be caching old non-existent user responses.
14 Jul 2022 - Email Geeks
Marketer view
Marketer from Email Geeks explains that the '550 5.1.1 Recipient address rejected: User unknown' error is highly likely to be generated by the recipient's Mail Transfer Agent (MTA), indicating it cannot find the mailbox. This suggests a bad address or a recipient-side misconfiguration, and should be treated as an invalid address.
27 Oct 2023 - Email Geeks
What the experts say
2 expert opinions
The '550 5.1.1 Recipient address rejected: User unknown' bounce error is a definitive indicator of a non-existent or unconfigured recipient email address. This is uniformly classified as a hard bounce, signifying an irreversible delivery failure. Experts concur that such an error means the address should be removed from mailing lists due to issues like misspelling or the user no longer being associated with that specific address.
Key opinions
- Confirms Invalid Address: This specific error code explicitly confirms that the recipient email address does not exist or is not actively configured on the destination mail server.
- Hard Bounce Indication: Designated as a hard bounce, this issue indicates a permanent and unrecoverable delivery failure, meaning the email will never be successfully delivered to that address.
- Various Root Causes: Common causes for this bounce include a misspelled email address, an individual having departed from their company, or the address simply never having existed or been active.
- Recipient Server Rejection: The error is generated by the recipient's mail server, which cannot locate the specified user account, thus rejecting the message outright.
Key considerations
- Prompt Address Removal: Given that '550 5.1.1' signifies a permanent failure, the affected email address must be promptly removed or suppressed from your mailing list to prevent future bounce errors.
- Maintain List Hygiene: Regular list hygiene, including verification of addresses and removal of old or invalid entries, is crucial to prevent these types of bounces and maintain deliverability.
- Protect Sender Reputation: Persistently attempting to send emails to non-existent addresses will negatively impact your sender reputation, potentially leading to future deliverability issues with other recipients.
Expert view
Expert from Spam Resource explains that '550 5.1.1 Recipient address rejected: User unknown' errors indicate the recipient email address does not exist or is not configured to receive email. This is a hard bounce, meaning the address should be removed from your mailing list.
16 Oct 2021 - Spam Resource
Expert view
Expert from Word to the Wise explains that '550 5.1.1 Recipient address rejected: User unknown' is a permanent failure (hard bounce) commonly indicating an invalid recipient. This occurs if the email address does not exist, is misspelled, or the person has left the company. Such addresses should be removed from mailing lists.
13 Jan 2025 - Word to the Wise
What the documentation says
5 technical articles
The '550 5.1.1 Recipient address rejected: User unknown' email bounce consistently points to one core issue: the specified recipient email address does not exist at the destination domain or on its mail server. This is widely understood as a hard bounce, representing a permanent and unrecoverable delivery failure. Mail service providers and experts agree that such an error means the address should be promptly removed from sending lists.
Key findings
- Non-Existent Address: The '550 5.1.1' bounce code unequivocally indicates that the recipient's email address is invalid, non-existent, or simply unknown at the destination mail server.
- Permanent Delivery Failure: This error is categorized as a hard bounce, meaning it signifies a permanent and unrecoverable delivery failure for the specific email address.
- Recipient Server Origin: The '550 5.1.1' error originates directly from the recipient's mail server, which cannot find a corresponding user account for the specified email address.
- Common Triggers: Frequent causes include typographical errors in the address, the recipient's account being deleted or inactive, or the address never having existed on the domain.
Key considerations
- Crucial List Hygiene: To mitigate these bounces, regularly verify and clean your email lists by removing invalid, outdated, or non-existent addresses.
- Suppress Invalid Addresses: Upon receiving a '550 5.1.1' error, immediately suppress or remove the bounced email address from your mailing list to prevent future delivery attempts.
- Safeguard Sender Reputation: Continuously sending emails to addresses that consistently return '550 5.1.1' errors can negatively impact your sender reputation and lead to broader deliverability issues.
- Broader Causes to Consider: While often a typo, this error can also signal a recipient has left their organization, their account is no longer active, or, in rare instances, the account has not yet been fully provisioned on their server.
Technical article
Documentation from Microsoft Learn explains that a '550 5.1.1 User unknown' error indicates the recipient email address does not exist at the destination domain. This is a common non-delivery report (NDR) in Exchange Online and Office 365, signifying a permanent failure.
7 Mar 2022 - Microsoft Learn
Technical article
Documentation from Postmark identifies the '550 5.1.1' error as a hard bounce, indicating that the recipient's email address is invalid or non-existent on the mail server. It advises checking for typos or removing the address from the mailing list.
20 Jan 2023 - Postmark
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