If you receive any of these messages when trying to send email, it would appear that outgoing mail is being blocked because your IP address (the dynamic address used to identify your computer on the internet) is appearing in a blacklist. If the IP address listed in the error begins 79.170.4 please contact support, otherwise read the information below. 

You can see more information by 
1) Finding your IP address from http://www.whatsmyip.net/ 

If you are receiving either: 
550 IP in XBL, 
550 IP in SBL, or 
550 IP in PBL, 

2) Entering this IP address into the Spamhaus advisory page at:- 
http://www.spamhaus.org/lookup.lasso 

If you are receiving either: 
550 IP in SCBL or 
550 IP in RBL 
3) Entering this IP address into Spamcop at:- http://www.spamcop.net/bl.shtml 

It's appearing in this blacklist because either your computer, or a previous computer using the same dynamically assigned IP address, may have been infected by a trojan or virus, taking advantage of your computer to send spam. 

Although this could just be a false alarm, caused by a previous computer on the same IP, we would recommend running a full virus, trojan and spyware scan on your computer - as well as ensuring you're using an active firewall. 

Once you're sure your system is clean, there are details at the above link as to how you can remove yourself from the blacklist. Be aware though, if you are still affected, you will be relisted very quickly. 

If you have a Dynamic IP Address it is possible that another machine that used the IP caused it to become blacklisted, in that situation restarting your modem / router can allow your ISP to issue you with a new IP.