450 / 451 4.7.1: Greylisted – try again later

AnySMTP

The error usually looks like this:

451 4.7.1 Greylisting in action, please come back later

What it means

The receiving server temporarily refused the message to check that the sender retries, as real mail servers do. It is not a permanent failure.

Common causes

  • The receiving server uses greylisting for senders it hasn’t seen before.

How to fix it

  1. No action is usually needed – the sending server retries automatically and the message is accepted.
  2. If you are sending directly from a script, make sure failed messages are retried after a few minutes.

Tools to help diagnose it

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