User specific rulesets

[Burak Ueda]

Greetings everyone.
I have a question about rule sets.
First let me describe the environment and conditions:

Server:
Intel Dempsey Dual Core HT 3.0 GHz (4 virtual processors)
2 GB of memory (upgradeable to 16 GB)
CentOS 4.5 / Apache 1.3
cPanel/WHM control panel installed.

This is an email server. There are about 80 domains (with no websites) 
and currently a bit more than 2000 email accounts.
Number of email account expected to go up to 10K in this 1~2 years.
The busiest domain will have approx 500 email accounts, the domain with 
the least email accounts will have around 50.

I will let the email account owners write their own filter, up to 10 per 
address. This will be black list only.

Here is a simple scenario:

Mr. A has an address: email-A at domain1.com
Mrs. B has an address in same domain:  email-B at domain1.com

These two people are not related to each other in way. They just own an 
email address with same domain name. Just like gmail.com, yahoo.com etc.
So Mr A doesn't wants to receive email from  offers at shop.com,
but Mrs. B is a regular customer of shop.com and want to get emails form 
offers at shop.com

I can achieve this by editing the global rules file:
/usr/mailscanner/etc/rules/spam.blacklist.rules
adding this line:
To: email-A at domain1.com and From: offers at shop.com    yes

Here is the problem:
Lets say server has 9000 email accounts. And 70% of the email account 
owners are using personal filter.
It is unlikely that all the users will use all 10 filters, so average 
filter usage per user will be like 5.
With some calculations, my global black list file will have 30K to 40K 
lines of rules.
And each time an email arrives to the server, MailScanner will check 
this huge blacklist file (correct?).

Instead, I want to do this:
Create a global blacklist file with 50 or so line, included the most 
obvious spammers.
And create a rule set for each domain. Somewhere in:
/home/user/etc/spam.blacklist.rules

So even for the busiest domain, even if the all account holders using 
all of their 10 rules, it will make 5000 lines of rules.
So MailScanner will check the global file 50 lines + black list file for 
the receiver domain 5000 = will be 5050 lines max.

I am so sorry for over detailed explanation, but I hope you got my point.
Is this possible at all? If yes How?
And if this is not possible, is my server specs enough to handle that 
big blacklist file ?

Thanks in advance.