TL;DR
Nonexistent recipient mailbox: do not retry unchanged; correct or retire that address.
What this code means
X.1.1 means the specified mailbox does not exist; for an Internet address, the invalid part is left of the @ sign. The standard reserves this detail for permanent failures, and 5.1.1 is its class-5 form.
Provider examples
550 5.1.1 Account not found550 5.1.1 <${user@example.com}>: Email address could not be found, or was misspelled (G8)550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. Learn more at https://support.google.com/mail/?p=NoSuchUser - gsmtp550 5.1.1 The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces. For more information, go to Fix bounced or rejected emails. - gsmtpRemote host said: 554 delivery error: dd This user doesn't have a yahoo.com account (testing123@yahoo.com) [-5] - mta123.mail.mud.yahoo.comsmtp;550 cuda_nsu 5.1.1 <email@example.com>... User unknownsmtp;550 5.1.1 Not our Customersmtp;550 5.1.1 <example@comcast.net> recipient mailbox unallocatedsmtp;552 5.1.1 <example@comcast.net> recipient rejected - extended quota violation<sanjit@greytip.com>: host aspmx.l.google.com[142.250.27.27] said: 550-5.1.1 The email account that you tried to reach does not exist. Please try 550-5.1.1 double-checking the recipient's email address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1 https://support.google.com/mail/?p=NoSuchUser v11-20020a170906564b00b009fd6f428f93si881612ejr.222 - gsmtp (in reply to RCPT TO command)550 5.1.1 <example@icloud.com>: user does not existsmtp;550 5.1.1 <email@example>: Recipient address rejected: User unknownsmtp;550 5.1.1 <email@example.com>: Recipient address rejected: undeliverable address: host example.com[x.xx.xx.xx] said: 550 5.1.1 <email@example.com>: Recipient address rejected: user unknownsmtp;550 5.1.1 - This is not a valid email address. Please verify that you have the correct email address for your recipient.Technical meaning
The receiving system rejected the destination mailbox as nonexistent. This is an address-level condition, not a statement about every mailbox at the domain.
Delivery status
Class 5 makes the current request permanent. An unchanged retry to the same nonexistent mailbox is not expected to succeed.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Stop automatic and manual retries to the unchanged address. Use a corrected address only when independently verified.
Suppression decision
An exact, confirmed 5.1.1 can support address-specific list hygiene under local policy. Do not suppress the entire domain or infer that unrelated recipients are invalid.
Common causes
- The local part names a mailbox that does not exist at the destination.
Diagnostic steps
- Confirm the exact code, address, response text, and transaction time before correcting or retiring the address.
Actions by owner
Sender
- Stop delivery attempts to that unchanged address and request a corrected address through a trusted channel.
Sender administrator
- Record the address-specific outcome and apply the organization’s hygiene policy without expanding it to the whole domain.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- Why the message was not delivered — Interia Mail help article explaining delivery-failure responses.
- Common email bounce messages — Rackspace documentation of common SMTP bounce messages.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- Gmail SMTP errors and codes — Official Gmail Help table of SMTP error messages and status codes.
- Yahoo SMTP Error Codes — Yahoo Sender Hub guide to SMTP deferral and rejection codes.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- email-bounce-parser (test fixture corpus) — Open-source bounce-parsing library whose test fixtures are pinned locally as evidence.
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