EC
455
SMTPWARNNotableTemporary Command FailureHIGH confidence

Server unable to accommodate parameters

What this means

The 455 code indicates a temporary failure where the server cannot handle the specific parameters of a command right now. The command itself is valid, but the current server state prevents it from processing the request as-is.

Why it happens
  1. 1The combination of `MAIL FROM` and `RCPT TO` parameters is temporarily disallowed by a server policy.
  2. 2An ESMTP parameter (e.g., related to DSN) is valid but cannot be fulfilled at this moment.
  3. 3The server is in a state where it cannot accommodate the request, but might be able to later.
How to reproduce

A client requests a delivery status notification (DSN) feature that the server is temporarily unable to provide.

trigger — this will error
trigger — this will error
RCPT TO:<user@example.com> NOTIFY=FAILURE
455 4.4.4 Server unable to accommodate NOTIFY parameter

expected output

455 Server unable to accommodate parameters

Fix

Retry without optional parameters

WHEN When the feature is not critical

Retry without optional parameters
// If a 455 occurs on a command with optional ESMTP params, retry without them
if (smtpCode === 455) {
  retryCommand(command.withoutOptionalParams());
}

Why this works

This allows the core mail transaction to proceed if the optional extension was the cause of the temporary failure.

What not to do

✕ Treat as a permanent failure

This is a transient error. The server is explicitly stating it *might* be able to process the request later.

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