Black List Removal Request

My company is receiving the following error message from several customers:

#553 5.3.0 alph160 DNSBL:ATTRBL 521< 72.11.91.102 >_is_blocked.For assistance forward this email to abuse_rbl@abuse-att.net

Neither http://rbl.att.net/block_inquiry.html or http://rbl.att.net/cgi-bin/rbl/block_admin.cgi load, and emailing abuse_rbl@abues-att.net has yet to fix the issue.

Please whitelist the IP: 72.11.91.102