Althouogh it may not be your problem, we should all be aware that there is a major issue with blacklisting by a agency called SORBS used by some ISP's to block unwanted spam. It lists whole blocks of IP adresses indiscriminately and it is nearly impossible to get them removed. The only solution is to complain to all ISPs and get them to stop using SORBS.

It’s easy enough to check if an IP is listed with SORBS at a Windows command prompt.

If we ping the mail server, it comes back with, say, an IP of 184.71.102.58. If we reverse the IP we can do a lookup on SORBS with the following;

nslookup 58.102.71.184.dnsbl.sorbs.net

The important bit is what comes back from sorbs. The Address: 127.0.0.10 is their code for a dynamic IP. They don’t care if any spam has ever come from that address or not. If they think it is dynamic IP space it’s automatically listed.

There is a rather lengthy six part article posted here about the evil ways of SORBS.

http://blog.wordtothewise.com/2010/12/gfi-sorbs-considered-harmful/