Thunderbird (found on 2.0.0.6) has a limit on the size of each folder (aka mbox file) of 2GB. It says the limit is 4GB on the thunderbird site, but this is not true. See https://bugzilla.mozilla.org/show_bug.cgi?id=207400
Thunderbird will not display any emails once this size is reached. The attached perl script will split the mbox files into 400MB files.
Then rename the files to something like Inbox-old1 Inbox-old2 and place them inside the Mail directory. Thunderbird will find them and index them like folders.
The source of the script is http://use.perl.org/~Purdy/journal/30632
-- PorchLight - 06 Oct 2007