The block message is 521 DNSRBL: Blocked for abuse

Bill Woodcock woody at
Wed Sep 18 17:15:02 UTC 2013

On Sep 18, 2013, at 9:07 AM, NANOG <NANOG at> wrote:
> We recently purchased new IP addresses from ARIN

No, actually, you didn't.  You were assigned the use of the addresses, based on need.  Just as a radio station does not "purchase" spectrum.

> Our mail server IP address is  Is it possible they are blocking us based on old information from the previous IP address block owner?

Quite likely, yes.

John LeCoque's suggestions seemed quite sound:

> I would say the first step is to find an immediate workaround for your end
> users - maybe bring up a VM on AWS or some other cloud provider to use as
> an SMTP relay while you work out the blacklist issue. If you run into
> blacklist issues after that, you may want to take a very close look at your
> outbound mailflow to make sure your organization isn't sending out messages
> that could be categorized as spam.


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