Whilst trying to send email to a GEN Customer you may receive a non-delivery report mentioning error 554. Something like:

554http://www.gensupport.net/Diag/blacklist.asp?i=92.42.145.68

This indicates that your email server is listed in one or more blacklists and will not be accepted. By Following the link provided you can see exactly which blacklists list you. In the example above (which is a real example) we see that....

0 | Spamhaus Block List = OK
1 | Exploits Block List = LISTED
0 | The Policy Block List = OK
0 | SpamCop Blacklist = OK
0 | MAPS Relay Spam Stopper (MAPS-RSS) = OK
0 | Distributed Server Boycott List (DSBL) = OK
0 | Spam and Open Relay Blocking System = OK
0 | Barracuda Extortware Blacklist (Not Used by GEN) = OK

The IP Address is listed in the exploits blacklist at Spamhaus.org. The XBL (which is what we call this Exploits Black List) is a realtime list of hi-jacked and compromised hosts on the internet. The fact that this IP address is listed means that host has been compromised by malicious software and cannot be trusted. This is the reason it is refused from delivery. 

See the blacklists FAQ for more generic information on blacklists.