SMTP error 510
Invalid recipient address — a vendor label, not a real code.
Updated Jul 1, 2026
SMTP 510 is a vendor label for an invalid or nonexistent recipient address, not a code defined in RFC 5321. The standards-correct equivalents are 550 with enhanced status 5.1.1 (bad destination mailbox) or 553 (mailbox name not allowed). The fix is the same either way: correct or remove the bad recipient in your To, Cc, or Bcc, since a retry never succeeds.
Correct or remove the recipient address; read the real code in the bounce.
What it means
There is no 510 reply code. RFC 5321 section 4.2.3 lists the permanent-failure codes as 500, 501, 502, 503, 504, and 550 through 555, with 521 and 556 added later by RFC 7504. 510 is in none of them. The structure is also irregular: in RFC 5321 section 4.2.1 the second digit classifies the reply, where x1x is informational and x5x is mail system status, so a real permanent recipient-address failure comes back as a 5xx mail-system code, most often 550 or 553. Treat 510 as a vendor synonym for "bad recipient" and read the full bounce text to find the real code your server sent.
Common causes
The local part before the @ has no account on the receiving server. Standards code: 550 5.1.1, "bad destination mailbox address" (RFC 3463 section 3.2).
A typo, an illegal character, a missing domain, or a name the server refuses. Standards code: 553 5.1.3 ("bad destination mailbox address syntax") or 501 ("syntax error in parameters").
No valid MX, or the domain rejects mail. Standards code: 5.1.2 ("bad destination system address") generally, or 556 with X.1.10 ("null MX") when the domain explicitly refuses mail via a null MX record.
Addresses that were valid once but have since been abandoned, or were entered incorrectly at capture.
How to fix it
- Read the full bounce line, not just "510"
Find the real reply code and enhanced status, for example 550 5.1.1 versus 553 5.1.3. That tells you whether the mailbox is wrong (550 5.1.1), the syntax is wrong (501 or 553), or the domain is wrong (5.1.2). The remedy differs.
- Verify every recipient in To, Cc, and Bcc
Check for typos, leading or trailing spaces, smart quotes, and missing or wrong domains. A single bad address in the envelope can fail the whole transaction.
- Validate before sending in bulk
Run addresses through syntax and MX validation and remove ones that do not resolve. This prevents repeated 550 5.1.1 bounces that damage sender reputation.
- Suppress hard bounces
Once an address returns a permanent invalid-mailbox result, add it to a suppression list and stop mailing it. Continuing to hit known-bad addresses raises your bounce rate and hurts deliverability.
Paired code
A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:
Common questions
Is SMTP Error 510 a real SMTP code?
No. RFC 5321 section 4.2.3 does not define a 510 reply code; its permanent-failure codes are 500 through 504 and 550 through 555, with 521 and 556 added later by RFC 7504. "SMTP 510" is a vendor label for an invalid recipient address. The equivalent standards codes are 550 5.1.1 (bad destination mailbox) or 553 (malformed address).
What is the difference between SMTP 510 and 511?
Both are non-standard vendor labels pointing at recipient-address problems. Neither is in RFC 5321. In each case the useful step is to read the real 5xx code and enhanced status in the bounce and act on that.
Can I retry a message that returned SMTP 510?
No. The underlying failure is permanent, so the same address bounces again. Fix or remove the recipient rather than resending.