Size limitation on Spam Whitelist

[#1]

As a business, we wish to whitelist all the people we correspond with, to prevent the false positives were are still getting. However, after trying to upload our list, we have been told by support that the whitelist can be a maximum of 1000 characters in length.
For a home user, I guess being able to whitelist around 50 email addresses or domains may be sufficient, but for a business, this is totally useless. Having been through the address books of the staff, and removed duplicates, we have around 750 addresses or domains we regularly correspond with. (Around 250 domains, and 500 specific address which can't be specified as a domain i.e. xxx@aol.com)
Given that Postini is a service aimed at business users, I can't believe this is a system limitation at Postini.
Can anyone enlighten me as to the real issue here?

[#2]

How about http://email.about.com/od/emailbehindthescenes/f/address_length.htm
The maximum theoretical length of a single email address is 320 characters 

[#3]

From reading Postini online documentation it was my understanding that each of the sender lists (white and black) can be a maximum of 4000 characters which covers just over 100 entries of typical length. However that restriction might apply for different service options than PlusNet pay for.
So I assume the information you were given by support is correct.