EC
553
SMTPERRORCommonDelivery FailureHIGH confidence

Requested action not taken: mailbox name not allowed

What this means

The 553 code indicates that the syntax of the mailbox name is correct, but the name itself is not allowed. This is often due to a policy on the server, such as requiring a fully qualified domain name.

Why it happens
  1. 1The `MAIL FROM` address is not a fully qualified domain name (e.g., `user@localhost`).
  2. 2The server requires the `MAIL FROM` address to be from a domain it considers valid.
  3. 3The syntax is correct but violates a specific local server policy.
How to reproduce

A client attempts to send mail with a non-fully-qualified sender address.

trigger — this will error
trigger — this will error
MAIL FROM:<sender@hostname>
553 5.1.8 Domain of sender address sender@hostname does not exist

expected output

553 Mailbox name not allowed

Fix

Use a Fully Qualified Domain Name (FQDN)

WHEN When configuring the sending application or server

Use a Fully Qualified Domain Name (FQDN)
// Ensure the sender address is a valid, FQDN
const sender = "user@example.com"; // Good
const badSender = "user@localhost"; // Bad

Why this works

Many servers reject mail from non-routable or unqualified domains to combat spam.

What not to do

✕ Retry without changing the sender address

The address format has been rejected by a server policy; it will be rejected again.

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