451 4.4.1 error from certain domains


  • I look after around 25 machines and a mail server where I work.
    A few months back we changed ISP and since then we've had trouble we email.
    We found out that most of it was down to us not having exchange configured correctly, but a couple of our customers are still not able to send us emails or reply to us.

    I've managed to get them to fax us a copy of the NDR

    The original message was recieved at wed, 17 sept 2008 19:20:01 +0100 (bst) from mail.sender.com

    -----transcript of session follows------
    451 4.4.1 reply: read error from mail.reciever.com
    <recipient@reciever.com>... deferred: operation timed out with mail.reciever.com
    warning: message still undelivered after 4 hours will keep trying until message is 5 days old.

    This message was sent to our customer from the send smtp gateway on out mx records.
    does anyone have any suggestions?

    Friday, September 19, 2008 7:58 AM


  • Hi,

    Error 451 4.4.1 looks like a temp error, it indicates that message will retry to be delivered.

    I would like to know whether several domain cannot send email to your organization or just one domain.

    Please try to use command to send email to user in your environment.

    Telnet to Port 25 to Test SMTP Communication


    Besides, please use NSlookup to check your DNS settings.

    Using NSlookup.exe


    Besides, please check whether any “Return e-mail DNS check” setting has been set on your Exchange Server or Firewall.

    Is there any spam filter configured on your mail server?

    More information share with you:

    Cannot send or receive e-mail messages behind a Cisco PIX firewall


    Hope it helps.


    Monday, September 22, 2008 7:29 AM