SMTP Error 511

Is "SMTP Error 511" a real SMTP code?

No. There is no 511 reply code in RFC 5321, the SMTP specification. Valid three-digit replies use a constrained set of values, and in the 5xx (permanent failure) range the address-related codes are 550, 551, 552, 553, and 554 — not 511.

"511" is virtually always a misreading of the enhanced mail system status code 5.1.1 defined in RFC 3463. Enhanced status codes are written as three dot-separated numbers (class.subject.detail), and 5.1.1 often appears in a bounce line right next to the real reply code, like:

550 5.1.1 <jdoe@exaple.com>: Recipient address rejected: User unknown

Here 550 is the actual SMTP reply code and 5.1.1 is the enhanced detail. Strip the dots and "5.1.1" reads as "511" — which is where this label comes from. (Some legacy docs also call this a "duplicate of 510"; that framing is itself based on a non-standard code and isn't found in any RFC. Note that a few provider blog posts and error-code roundups also list "511" as if it were its own standalone reply code — it isn't; it's this same 5.1.1 mix-up.)

What does 5.1.1 / "bad destination mailbox address" mean?

Per RFC 3463, X.1.1 means: "The mailbox specified in the address does not exist. For Internet mail names, this means the address portion to the left of the '@' sign is invalid." The leading 5 marks it a permanent failure — resending the same message unchanged will not succeed. The recipient's server accepted your connection and recognized the domain, but rejected the specific local-part (the part before the @).

This is distinct from neighboring codes you may also see:

• 5.1.2 — the destination domain/system doesn't exist or can't accept mail.
• 5.1.3 — the address is syntactically invalid (malformed, not just unknown).

How do I fix "SMTP Error 511" (5.1.1)?

1. Read the full bounce line, not just the digits. Capture the real reply code (550 vs 553) and the human-readable text after 5.1.1 — providers append the actual reason ("User unknown," "Recipient address rejected," "No such user").
2. Correct the recipient's local-part. 5.1.1 points at the left of the @. Check for typos (jhon@, missing letters), stale usernames (employee left, alias removed), or a wrong-but-valid-looking address.
3. Confirm the mailbox actually exists. Ask the recipient for their current address, or send a test to a known-good address on the same domain to rule out a server-wide problem.
4. Remove the address from active sends. Because 5.1.1 is permanent, suppress or delete it from your list rather than retrying. Continuing to mail addresses that hard-bounce hurts sender reputation.
5. Validate at intake. Use double opt-in and address validation to catch invalid recipients before they bounce. A spike in 5.1.1 across many recipients usually signals bad list data (a scraped or aged list), not individual typos.