MailFixIt

Generic SMTP delivery response

Understand SMTP 5.1.3: Invalid recipient address syntax

The destination mailbox address is not valid SMTP address syntax.

Problem description

A representative response is:

501 5.1.3 Invalid recipient address syntax.
Is this temporary or permanent?

The same malformed address will fail again. Correct it before resending.

What this response means

Standard 5.1.3 Bad destination mailbox address syntax means the recipient address is malformed. The receiving server could not interpret it as a valid destination mailbox.

Fix it

  1. Copy the actual SMTP recipient from the log or NDR, not only the address displayed in the message.
  2. Check for a missing local part or domain, spaces, doubled @, pasted display-name text, invalid quoting, or hidden characters.
  3. Correct the address at its source: form input, customer record, mailing list, CRM, or application template.
  4. Make the application validate and encode mailbox addresses with a maintained email library.
  5. Generate a fresh message and verify the RCPT TO value before retrying.

Do not change DNS for a syntactically invalid address.

Who can fix it

The sender or application that produced the recipient value owns the correction.

If the syntax is valid but the mailbox is absent, use the 5.1.1 guide instead.

Frequently asked questions

What does SMTP 5.1.3: Invalid recipient address syntax mean?

The recipient address is malformed, for example because a required part is missing or invalid characters or display-name text entered the SMTP address.

Can MailFixIt prove this entire bounce from public DNS?

No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.

Related email diagnostics

SMTP 550 5.7.26: Multiple authentication checks failedUnderstand SMTP 5.7.26 and separate SPF, DKIM, DMARC, and alignment failures before changing email DNS.

Primary sources: IANA SMTP Enhanced Status Codes.