is there a limit to max whitelist & blacklist

[ram]

We use MS + SA + postfix for around 1000 domains 
Custom whitelists and blacklists are implemented  using ( in
MailScanner.conf ) 

Is Definitely Not Spam = %rules-dir%/spam.whitelist.rules
Is Definitely Spam = %rules-dir%/spam.blacklist.rules

and these files have  entries like 
------
From: domain1 and To: domain2 yes 
From: user at domain1 and To: domain3 yes 
.
.
.

-------------

I have around 40k entries in both files together


This had been working fine for over 3 years now but suddenly off late I
have been noticing that whitelists sometimes fail to get applied 

Unfortunately I am not able to reproduce the situation, but can someone
tell me if there is something obvious that can go wrong in this kind of
scenario



Thanks
Ram