How do I fix this?

["M A" <geneticflyer@googlemail.com>]

Hi there Got this from my ISP the other day

 

We have been forced to take your server off line, since your server is
performing phishing from your secondary IP address xxx.xxx.xxx.224.

 

that IP address was one my secondary IP's, using debian sarge, have iptables firewall,

using qmail as the mail server ..

 

How do i fix this, or detect that is happening ..

 

 

Cheers