SMTP Error 515 - Invalid recipient address — not a real RFC code. | Warmup Inbox

The short answer

SMTP 515 is not a real SMTP reply code. RFC 5321 defines no 515, so no standards-compliant server returns it. The "515 destination mailbox address invalid" label comes from error-list websites and points at an invalid or nonexistent recipient. The genuine codes are 550, 551, or 553 with enhanced status 5.1.x, so fix it by correcting the recipient address.

Quick fix

Correct the recipient address; act on the real code in the bounce.

5 - Permanent. The server will not try again. Fix the cause before you resend.

What it means

There is no 515 reply code in the SMTP standard. RFC 5321 section 4.2.3 lists the permanent-failure codes as 500, 501, 502, 503, 504, and 550 through 555, and 515 is not among them. Other registered codes such as the 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 compliant server never returns a bare 515. The "515 destination mailbox address invalid" wording comes from third-party SMTP error-list sites, and none cite an RFC because none exists. Treat any literal 515 as a non-standard label.

Common causes

Recipient does not exist (550 5.1.1)
- The standard no-such-user rejection, "requested action not taken: mailbox unavailable", carrying enhanced status 5.1.1, "bad destination mailbox address".
User not local (551)
- 551 "user not local; please try the forward-path" redirects mail to the correct host.
Malformed address (553 5.1.3)
- 553 "mailbox name not allowed" with 5.1.3, "bad destination mailbox address syntax", covers a typo, a stray space, smart quotes, or display-name leakage.
Domain problem (5.1.2)
- The domain is missing or cannot accept mail, for example a typo like gmial.com that fails DNS.

How to fix it

Read the real bounce in your provider logs
In SendGrid, Amazon SES, or Mailgun, find the actual permanent code and enhanced status. Ignore any 515 relabeling.
Validate the recipient address
Check 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.
Verify the domain resolves
Run "dig MX recipient-domain.com" for a 5.1.2 case. A typo such as gmial.com produces this.
Do not auto-retry
A permanent failure (RFC 5321 section 4.2.1) should be suppressed, since repeated hard bounces hurt sender reputation.
Do not confuse this with Microsoft 550 5.7.515
That is 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.

Paired code

A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:

X.1.1 Bad destination mailbox address

Common questions

Is SMTP Error 515 a standard SMTP reply code?
No. RFC 5321 section 4.2.3 lists every defined reply code and 515 is not one of them. The registered permanent-failure codes are 500 through 504, 521, 530, 534, 535, 538, 550 through 555, and 556. "515" is a label used by third-party error-list sites, not a code any compliant server returns.
What code does a server really send when a recipient address is invalid?
550 (mailbox unavailable), 551 (user not local), or 553 (mailbox name not allowed), usually with an enhanced status of 5.1.1, 5.1.2, or 5.1.3 depending on whether the mailbox, domain, or syntax is at fault.
Is this the same as Microsoft 550 5.7.515?
No. 550 5.7.515 is a real Microsoft 365 code for a DMARC or authentication failure, which you fix with SPF, DKIM, and DMARC. The "515" label here points at an invalid recipient address instead.