SMTP enhanced status code 5.1.3

What the registry says

The basic codes it travels with

• 501 Syntax error in parameters or arguments

In practice

X.1.3 bounces usually trace back to how the list was built: addresses pasted with trailing commas or quotes, two addresses in one field, a display name left in, or characters a form failed to strip. Some servers reject these at RCPT TO with 501 or 553 before any message is sent. Syntax errors are the cheapest bounce to avoid, since they can be found without contacting any server, and a list with many of them often has other quality problems as well.

What the sender should do

Correct the address if the intended one is obvious, otherwise drop it. Syntax checks at the point of capture stop these from entering the list at all.

How providers use X.1.3

Quoted from each provider’s own error reference.

• Gmail 553 5.1.3

  The recipient address address is not a valid RFC 5321 address. For more information, go to About SMTP error messages and review RFC 5321 specifications.

Common questions

What does 5.1.3 mean?

X.1.3 means the recipient address is not valid syntax, for example a missing @, spaces, two @ signs or illegal characters. The receiving server could not even parse it as an address, so it is permanent.

What should I do about 5.1.3?

Correct the address if the intended one is obvious, otherwise drop it. Syntax checks at the point of capture stop these from entering the list at all.

Would an email verifier have caught 5.1.3?

Yes, verification catches this. A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.

Full guides for this code

• 553 5.1.3 — invalid address syntax