Ive got some issue with email forwarding. Through the ISPConfig 3.0 i've set some email forwards. When I'm trying to send the email on the forward address daemon is returning the message with error 521, which mean that message is spam. But on the second hand, when i'm trying to send the email direct on the address without the forward - the same topic and content - the email easly go to the inbox. Where can be the problem? Whole error message:

<@ameritech.net> (expanded from <@forward-domain.com>): host aitmx9.prodigy.net[] said: 553 5.3.0 nlpi147 DNSBL:ATTRBL 521< ... >_is_blocked.__For_information_see_http://att.net/blocks (in reply to MAIL FROM command)

2 Answers 2


Some of the hosts involved are apparently listed on a black list (DNSBL).

I can't tell from your description what you tried to send how from where, but one of the hosts involved in the failed attempt is listed for whatever reason in some black list.

  • Okay, see you are right, i've checked the DNSB List and found that host is on SpamCannibal list because of invalid PTR record. So making a PTR record for mail dns records should be helpful in this case?
    – Kamil
    Dec 2, 2011 at 10:20
  • If this is indeed the only reason for the listing, it should be sufficient, but I guess you will have to wait until the listing expires or somehow contact them to remove your entry.
    – Sven
    Dec 2, 2011 at 10:36
  • It would be easier if my IP was banned, but it's whole OVH.. great =)
    – Kamil
    Dec 2, 2011 at 10:51
  • If the host has a dynamic IP address, it won't be able to get of some blacklists. As per the RFCs it needs a static address. Many ISPs have provided lists of dynamic addresses so that they can be blacklisted and not used to send spam.
    – BillThor
    Dec 2, 2011 at 13:15

DNSBL:ATTRBL 521< ... >_is_blocked.__For_information_see_http://att.net/blocks

This is a DNSBL (client) blacklist response.
It has nothing to do with addresses, i.e. nothing from that machine will be accepted by att.net.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .