I am running a private postfix server for my domain mydomain.com, on server (IP and domain sanitized for privacy reasons)

I have sent an email to [email protected] which is on server The message was deferred, and in my postfix logs, I could see following error:

postfix/smtp  9D055EA: host mail1.example.com[] said: 
451- is not yet authorized to deliver mail from 451 
<[email protected]> to <[email protected]>. Please try later. (in 
reply to RCPT TO command)

My domain has reverse DNS record set up, and I am using SPF record. Both are configured correctly, I can send emails all right, this is the first case when I have seen these errors.

This leads me to believe, that this error is caused by misconfiguration on the destination server. But since I am not sure what these errors mean, I would like if somebody could explain it.

  • Does the destination server have greylisting enabled? Jul 17, 2014 at 23:10
  • @Greg Hewgill - I don't know. The destination server is not under my control. Jul 18, 2014 at 10:38
  • Did the retry succeed after sometime?
    – clement
    Jul 20, 2014 at 10:57
  • @clement - I did not wait to see if it would have succeeded. I have removed the mail from queue and sent it from gmail instead. Jul 21, 2014 at 6:01

1 Answer 1


The message "... is not yet authorized to deliver mail from ..." is similar to a greylist implementation described in Adding Greylisting Support. If the destination server implement similar script, you can send your mail by flushing your queue to retry sending soon.

$ postqueue -f

The greylist implementation works as follows;

  1. If a mail come from an unauthorized host, the server replies 451 and save a entry including sender/recipient/IP/...
  2. When the host retry to send the mail, the server checks whether all sender/recipient/IP are matched. If matched, the server accept the mail.

I think this idea is come from most spammer implementation. It sends many mails to random recipient with random sender addresses. So it never send to the same recipient.

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