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553 5.1.3: the address is not valid

The address you sent to is malformed. It never reached a mailbox lookup, because it is not an address the server can parse.

Updated 1 min read

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553 5.1.3 is a permanent SMTP reply: malformed address. This page is part of our SMTP error code reference, where every code links to the RFC and to what Gmail, Outlook and Yahoo publish about it.

Kind
Permanent: sending the same message again fails the same way
Retry by hand
No, fix the cause first
In Sukhovei Mail
bounced, address suppressed

What 553 5.1.3 means

553 is "Requested action not taken: mailbox name not allowed (e.g., mailbox syntax incorrect)" [1]. X.1.3 is "Bad destination mailbox address syntax" [2].

An SMTP session in which Sukhovei Mail's submission server answers RCPT TO with an address that has no top level domain with 501 5.1.3 is not a valid recipient address, then accepts a valid address on the same connection
A real session with Sukhovei Mail's submission server on port 465: after authentication, RCPT TO:<ada@example> gets 501 5.1.3, and RCPT TO:<ada@example.com> on the same connection gets 250. The key is masked.

What Gmail, Outlook and Yahoo say

Gmail
"553 5.1.3 The recipient address <address> is not a valid RFC 5321 address." [3]

Outlook and Microsoft 365 and Yahoo do not list 553 5.1.3 on the error pages we read, so we do not guess what they mean by it.

Sources, read on October 14, 2026

  1. [1]RFC 5321, Simple Mail Transfer Protocol, section 4.2: www.rfc-editor.org/rfc/rfc5321#section-4.2
  2. [2]RFC 3463, Enhanced Mail System Status Codes: www.rfc-editor.org/rfc/rfc3463
  3. [3]Google, Gmail SMTP errors and codes: knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes

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Common causes

  • A display name or spaces passed as the address.
  • Two addresses joined into one field.
  • A domain with no dot, such as ada@example, from a mistyped or half filled form field.

How to fix it

  1. Validate addresses before sending and pass plain addresses only, such as user@example.com, without a display name.
  2. Log the address from the reply: the text after the code repeats exactly what the server received, so you can see which field was wrong.

How Sukhovei Mail reports it

When a recipient's server answers 553 5.1.3 to mail you send through Sukhovei Mail, the recipient becomes bounced and you get a message.bounced event with the server's reply in response and permanent: true. The message statuses page lists every state.

5.1.3 says the address will not work, so Sukhovei Mail adds it to the project's suppression list: the next send to it fails with recipient_suppressed before anything leaves. Hard bounces also count toward the automatic pause, which stops a workspace whose hard bounces reach 5% of at least 50 recipients in 24 hours, with at least 3 of them.

Sukhovei Mail's submission server answers a malformed recipient with 501 5.1.3 at RCPT TO, not 553: the reply repeats the address it received. The connection stays usable, so a valid address can follow on the same session, as the capture shows.

Through the API the same mistake is refused before anything is sent, with the error code invalid_address and the fix "Use a plain address such as user@example.com, without a display name". See the errors page.

Related codes

Seeing an API error rather than an SMTP code? Sukhovei Mail's own errors, each with a fix, are on the errors page.

Questions

Is 553 5.1.3 permanent?

Yes. The address itself is malformed; fix it before sending again.

Why did Sukhovei Mail answer 501 5.1.3 instead of 553 5.1.3?

Sukhovei Mail's submission server uses 501, "syntax error in parameters or arguments", for a recipient it cannot parse as an address. The enhanced code 5.1.3 is the same.

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