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Email Rejected by Comcast

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  • Email Rejected by Comcast

    We're running Exchange 2000. We host our own email server behind a Netgear firewall.

    Most emails appear to be flowing fine. However, I've received 2 of these NDR's today.

    The following recipient(s) could not be reached:

    '[email protected]' on 9/17/2007 4:47 PM
    There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
    < #5.5.0 smtp;551 not our customer>
    I had a similar rejection notice from one other domain as well. I've done some searching and i think it may have something to do with Reverse DNS lookup.

    I've tried telnet to comcast and starting a mail conversation. At MAIL FROM:<[email protected]> I get a 530 AUTHENTICATION REQUIRED message.

    I'm not sure how our Reverse DNS should look (I wasn't the one that set it up) so I am sorta stumped at the moment. If anyone can shed some light for me I would appreciate it.

  • #2
    Re: Email Rejected by Comcast

    how many IP addresses do you have? does your MX record point to the same IP as your incoming internet connection?

    for example, i have mail delivered to one IP. i have internet traffic routed thru a second IP. because of that, i must make sure that my SMTP traffic is routed thru the same IP as my MX records indicate.

    before i had the policy in place, i had some orgs that denied the mail because it was originating from a different IP than my MX...

    just one thing to check.
    its easier to beg forgiveness than ask permission.
    Give karma where karma is due...


    • #3
      Re: Email Rejected by Comcast

      I tested my Reverse DNS and it appears to be working fine.

      I think it was a non-existent user, because I've sent test emails to several other accounts and it went through fine.


      • #4
        Re: Email Rejected by Comcast

        I would recommend getting a subscription to It's a great resource and has several email tests to help troubleshoot these kinds of issues.