5.1.1Recipient mailbox does not exist

The specified recipient mailbox does not exist. This is a permanent failure, so do not retry delivery to the same unchanged address.

The X.1.1 pattern means that the mailbox specified in the address does not exist; for an Internet mail address, the part to the left of the “@” sign is invalid. The standard states that this status is useful only for permanent failures, and code 5.1.1 belongs to permanent class 5.

The leading digit 5 denotes a permanent failure. Repeating the same attempt with an unchanged address is not the appropriate response; a new send is justified only after the address has been reliably corrected.

Class

Permanent failure

Retry

Do not retry unchanged

Suppression

Check the full context

Do not automatically or manually retry delivery to the same unchanged address. Make a new attempt only when the address has been corrected from a trusted source; do not guess a missing or different mailbox name.

Do not apply unconditional or blanket suppression based solely on code 5.1.1. Confirm that the permanent rejection applies to this exact address, inspect the full response, and apply the appropriate policy for that recipient; do not suppress the entire domain or other recipients.

• The part of the address before the “@” sign does not correspond to any mailbox in the recipient system, for example because of a typo.
• The sending system used an outdated or incorrectly recorded recipient mailbox address.

1. Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 5.1.1 and that the basic reply is in the 5xx class.
2. Compare the full recipient address with a trusted source value, paying particular attention to the part before the “@” sign; do not try to guess the correct address.
3. Trace the address to its source in sender data and review the attempt history so the permanent rejection is tied to the correct address and further unchanged sends are stopped.

Verified provider examples for 5.1.1 appear below, from Interia Poczta and Rackspace Email. They show real provider practice for this response and do not redefine the standard 5.1.1 meaning or cover every occurrence.

550 5.1.1 Account not found

550 5.1.1 <${user@example.com}>: Email address could not be found, or was misspelled (G8)

Compare 4.1.1, which has the same X.1.1 meaning but a temporary class; 2.1.5, which confirms a valid address; 4.1.8, which concerns the sender system; and 5.1.3, which describes invalid destination-address syntax.

• 2.1.5Destination address is valid
• 4.1.1Temporary destination mailbox address problem
• 4.1.8Temporary sender system address problem
• 5.1.3Invalid recipient address syntax

The X.1.1 meaning and the permanent class of code 5.1.1 are standards facts verified against the IANA registry and RFC 2034, RFC 3463, and RFC 5248 on July 17, 2026. No accepted provider example is available; the retry, suppression, diagnostic, and owner-action guidance is operational, not standards or provider facts.

• iana-smtp-enhanced-status-codesT0 source

  Enumerated Status Codes / X.1.1

• rfc5248T0 source

  Section 2.1: registry fields and non-exclusive Associated Basic Status Code

• rfc2034T0 source

  Section 4: enhanced status class agrees with SMTP reply class

• rfc3463T0 source

  IANA registry reference for X.1.1

• ext-src-interia-5-1-1-01T1 source

  Article body, delivery-failure error list, entry heading "550 5.1.1 Account not found"

• ext-src-rackspace-5-1-1-02T1 source

  docs.rackspace.com/docs/common-email-bounces > 'SMTP error' table, row address-not-found (G8)

Last verified: 17 July 2026