In the past several days, we have seen a large number of both SBC Global and Comcast users with blacklist problems. Predominately, the host IP Addresses are showing up on the Sorbs Dynamic User and Host List (DUHL) blacklist. By extension, users are seeing bounce back messages indicating that their emails are blocked because their IP Address is on a blacklist. We have every reason to believe that these particular blocks are false positives stemming from Net Blocking. Net Blocking is a shotgun approach to blacklisting, where a large range of IP Addresses are blocked, presumably because they are near a known spam IP. Unfortunately, for the people we have spoken with, they are at the mercy of their respective ISP/Email Host to get the problem resolved...which is a frustrating exercise according to most. Incidentally, we have also seen a large number of emails blocked by the blacklists that SBC uses to curb spam...probably no connection, but interesting.
The only other reportable trend is a large number of Level 1 blocks from an obscure German blacklist- UCEProtect. A UCEProtect Level 1 block blocks a single IP addresses that have "either wrong or missing or generic reverse dns (PTR), or dialups, open proxy's, open relays, or which are using abusive techniques or which assigned to well known spammers." However, most of the people on the UCEProtect list that we spoke with did not meet these conditions. This leads us into an interesting discussion topic:
False positives are a significant problem with many widely used blacklists, which is just one reason why a strong inbound/outbound email filtering solution is a superior anti-spam tool.