Spam problem and live ip blacklist

[ibtisambutt]

Dear All Aslam-u-Alaikum,
I've approximately 20 pcs on lan and i am using dsl line for internet. My problem is that some one system is generating spams to internet and in reply my ip become blacklist (block).
Now i want to diagnose what system is creating problem and how get rid of it.

Regards
Ibtisam

[peer]

good question