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553 5.1.3: Invalid recipient address syntax

The recipient address isn't a valid email address under RFC 5321: bad characters, missing parts or broken formatting.

Bounce type
Hard bounce (permanent)
What to do
Suppress the address
Returned by
Gmail / Google Workspace; Any mail server

What 553 5.1.3 means

5.1.3 is 'Bad destination mailbox address syntax': the address is malformed, so the server can't even evaluate whether the mailbox exists. Gmail's text says the address 'is not a valid RFC 5321 address'.

This one is almost always a data-quality problem in your list, not a deliverability problem. But it still counts as a failed send, and a list full of these is a sign the rest of the data is unreliable.

What the bounce looks like

The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.

Gmail
553 5.1.3 The recipient address [address] is not a valid RFC 5321 address.

Why cold emailers see 553 5.1.3

  • Spaces, commas, semicolons or quotes inside the address (often two addresses in one cell).
  • Missing @ or domain, or a double @.
  • Placeholder values such as 'N/A', '-', or 'firstname@' left in a CSV column, or spreadsheet errors like '#N/A' exported as text.
  • Non-ASCII characters in the local part sent to a server that doesn't support internationalized addresses.

How to fix 553 5.1.3

  1. Validate syntax on import and reject anything that isn't a single, well-formed address.
  2. Split cells containing multiple addresses into separate contacts.
  3. Fix or drop the contact; resending the same string will fail every time.

Retry or suppress?

Suppress (or correct the address). Never retry the same malformed string.

Reading the code

The basic code 553 means "Mailbox name not allowed (permanent)" under RFC 5321. In 5.1.3, the 5 is the class (permanent failure) and the 1 is the subject (addressing). The detail number 3 is registered in RFC 3463 / the IANA registry as "Bad destination mailbox address syntax".

Class 5
Permanent failure
Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
Subject 1
Addressing
A problem with the sender's or recipient's address: syntax or validity.
Detail 5.1.3
Bad destination mailbox address syntax

Check the rest of your setup

Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.

Got a different bounce? Paste it into the SMTP error code lookup to decode it.

FAQ

Is 553 5.1.3 a hard bounce or a soft bounce?

553 5.1.3 is a permanent failure (a hard bounce): the receiving server won't accept this message if you send it again unchanged. Here the problem is the address itself.

Should I remove the contact after a 553 5.1.3 bounce?

Yes. Suppress (or correct the address). Never retry the same malformed string.

What does 5.1.3 mean in the error code?

In 5.1.3, the 5 is the class (permanent failure) and the 1 is the subject (addressing). The detail number 3 is registered in RFC 3463 / the IANA registry as "Bad destination mailbox address syntax".

Related SMTP codes

See all SMTP error codes

Sources

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Clement reads the bounces that land in your Gmail and SMTP inboxes and tells dead addresses apart from policy blocks: it suppresses the first across every campaign and keeps the contact on the second, so you don't have to decode bounces by hand.

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