Please, Help me Tweak SpamAssassin...

I have a VPS hosted at powervps

The Plan is LINUX PLESK POWER-1, 256Mb (burstable to 1Go), 10Gb space

I'm currently allowed to configure spamassassin per server or per mailbox.

I would like the following;

I want to be a spam-filtering gateway for one of my customers.

But with the less effort possible.

Then a tool called PopCon to extract mail from catch-all account
then forward to the lotus domino server inside the client network

But problem with catch-all account is that it shred the BCC field.

So now that i have a VPS, i think i will be able to tweak SpamAssassin and QMail to do something different.

May i just be a spam-filtering gateway ?

May i just filter the mails then forward them to another ip address ?

Plus mails identified as spam could go to let's say another email on another domain. is filtered then forwarded to ip of customer domino server
Spam mails are forward to let's say

And i put a rule on to delete mails older than 5 days each night.
So this mailbox stay not too big and i always have spammed emails from the last 5 days.

Do i ask too much ?

Is it possible to do this ?