qmail and plesk environment - getting listed on spamhaus

[Johnny Stork]

I have a client running a plesk/qmail hosting service who is having some 
trouble with getting their shared ip listed on spamhaus. I am looking 
into various solutions to suggest to them, possibly including ms if it 
can be integrated into the plesk environment. For now I would like to 
try and determine why they keep getting listed on spamhaus. Can anyone 
suggest some tips or a starting point to determine why they might keep 
getting listed?

I will be reviewing their maillogs to see if there is any mass mailings 
going out from any of their hosted domains on the shared ip to start.