There were various "blacklist.zap" lists and they were all indicative of blocking when sending to mailboxes hosted behind "FrontBridge" anti-spam and security protection:
- The list 85.blacklist.zap specifically referred to FrontBridge's use of the Composite Blocking List (CBL). If you were blocked by 85.blacklist.zap, it meant that your sending IP address was listed on the CBL.
- The list 86.blacklist.zap specifically referred to FrontBridge's use of the Spamhaus Block List (SBL). If you were blocked by 86.blacklist.zap, it meant that your sending IP address was listed on the SBL.
- The list 87.blacklist.zap specifically referred to FrontBridge's use of the Spamhaus Exploits Block List (SBL). If you were blocked by 87.blacklist.zap, it meant that your sending IP address was listed on the XBL.
- The list 88.blacklist.zap specifically referred to FrontBridge's own internally-generated blacklist of sending IP addresses noted to be spammy, usually based on a high percentage of mail from that IP address being denoted as spammy.
FrontBridge was later acquired by Microsoft and I think it's been a long time since anybody has seen blacklist.zap blocking in a bounce message, but I thought it would be good to keep a record of this for posterity's sake.