EC
451
SMTPWARNNotableTemporary Server ErrorHIGH confidence

Requested action aborted: local error in processing

What this means

The 451 code indicates a temporary server-side error has occurred. The problem is with the receiving mail server, not the sender or recipient, and the client should try again later.

Why it happens
  1. 1The mail server encountered an unexpected internal error.
  2. 2A connection to a backend database or filesystem failed.
  3. 3Anti-spam or anti-virus scanners are timing out or failing.
How to reproduce

A mail server's anti-virus scanner crashes while inspecting an incoming email.

trigger — this will error
trigger — this will error
DATA
354 Go ahead
[...email data...]
.
451 4.3.0 Local error in processing

expected output

451 Local error in processing

Fix

Retry delivery later

WHEN When operating a sending client

Retry delivery later
// A 451 is a server-side transient failure, so retry
if (smtpCode === 451) {
  scheduleRetry(email);
}

Why this works

The error is on the server side and likely to be resolved, so retrying is appropriate.

What not to do

✕ Stop retrying after one attempt

Server-side issues can take time to resolve; a proper retry schedule (e.g., over several hours or days) is necessary.

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