Actualy... each custom box can contain 200 messages and you can have up to 4 custom boxes. Including the inbox that's a total of 1000.
Only reason I limit it is because if I didn't people come and go and all those messages would add up to a lot of space especially if they are using the attachment feature never to be read again. I'd imagine there's many MB's of unread very old messages from members that no longer come here. Probably have to do some purging at some point in time. The text don't add up to a huge amount but it will over time.