SMTP Error 515

Is SMTP Error 515 a real SMTP code?

No. There is no reply code 515 in the SMTP standard. RFC 5321 section 4.2.3 lists the base SMTP reply codes; its permanent-failure codes are 500, 501, 502, 503, 504, 550, 551, 552, 553, 554, and 555, and 515 is not among them. Other registered codes (such as AUTH codes 530, 534, 535, and 538 from RFC 4954, and 521 and 556 from RFC 7504) live in the IANA registry, where 515 also does not appear. A standards-compliant server never returns a bare 515.

The 515 destination mailbox address invalid wording comes from third-party SMTP error-list sites. None cite an RFC, because none exists. Treat any literal SMTP Error 515 as a non-standard label, not a real server response.

What is 515 pointing at?

It is grouped with the equally non-standard 513 and 517 entries and describes one real problem: the recipient address is invalid, malformed, or does not exist. That is reported by legitimate codes:

• 550 Requested action not taken: mailbox unavailable (the standard no-such-user rejection).
• 551 User not local; please try the forward-path.
• 553 Requested action not taken: mailbox name not allowed, or ambiguous address.

These usually carry an RFC 3463 enhanced status:

• 5.1.1 Bad destination mailbox address (recipient does not exist).
• 5.1.2 Bad destination system address (domain missing or cannot accept mail).
• 5.1.3 Bad destination mailbox address syntax (malformed address).

How to fix the invalid-recipient problem

1. Read the real bounce in your provider logs (SendGrid, Amazon SES, Mailgun) and find the actual permanent code and enhanced status. Ignore any 515 relabeling.
2. Validate the recipient address for typos, stray spaces, smart quotes, or display-name leakage (5.1.1 or 5.1.3). In bulk sends, one bad row is the usual culprit.
3. Verify the domain resolves with dig MX recipient-domain.com (5.1.2). A typo such as gmial.com produces this.
4. Do not auto-retry. A permanent failure (RFC 5321 section 4.2.1) should be suppressed, since repeated hard bounces hurt sender reputation.
5. Do not confuse this with Microsoft 550 5.7.515, a real, separate rejection: Microsoft 365 uses 5.7.515 for a DMARC or authentication failure, not a missing mailbox. Fix SPF, DKIM, and DMARC alignment (RFC 7489), not the recipient.