MailFixIt

Generic SMTP delivery response

Understand SMTP 5.1.7: Invalid sender address syntax

The message contains a malformed sender mailbox address that the SMTP system cannot accept.

Problem description

A representative response is:

550 5.1.7 Invalid sender address syntax.
Is this temporary or permanent?

The same malformed sender will be rejected again. Correct the generated address before retrying.

What this response means

Standard 5.1.7 Bad sender mailbox address syntax means a sender address is malformed. Depending on the server and wording, that can be the SMTP envelope MAIL FROM value or a sender field in the message.

Fix it

  1. Capture the SMTP transaction and raw message to identify the exact sender value that failed.
  2. Check for missing address parts, doubled @, spaces, display-name text in the envelope, invalid quoting, or hidden characters.
  3. Correct the source field in the application, bounce-address configuration, CRM, website form, or mail library.
  4. Preserve the valid null reverse-path <> for delivery-status messages; do not replace it merely because it looks empty.
  5. Generate and inspect a fresh message before retrying.

DNS authentication cannot repair invalid mailbox syntax, although a later delivery may still require valid SPF, DKIM, and DMARC.

Who can fix it

The application or outbound-mail administrator that constructs the sender address owns the repair.

Use sender-domain DNS guidance only if the corrected address then fails a separate domain or authentication check.

Frequently asked questions

What does SMTP 5.1.7: Invalid sender address syntax mean?

A sender mailbox address is syntactically invalid. Inspect the envelope MAIL FROM and the message sender fields to find the malformed value.

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.