The block message is 521 DNSRBL: Blocked for abuse

Wed Sep 18 17:47:57 UTC 2013

> > Our mail server IP address is 74.112.99.25.  Is it possible they 
> are blocking us based on old information from the previous IP 
> address block owner?
> 
> Quite likely, yes.

https://www.arin.net/resources/whowas/

Found it to be of use for this type of question. Registration required. 
Geolocation information often needs updating for these "recycled" ranges. 

/JF