mbox file too large, mailer is clunky

[darkweb4]

Right now I am using Mozilla 1.5 on a windows machine for my
email and newsgroup mailer.  I like to save all of my email,
and now I have several months of email built up in a 30-40
MB mbox file.  Everytime I access this file, it takes almost
a minute for Mozilla to display my mail.  Is there a leaner
mailer to use, one that could access this file faster? 
Alternativly, is there a way to easily split the file into
seperate months, that way Mozilla only has to load a few MB
to let me read my mail?  If I split the file into pieces, am
I going to be able to search all of the files cocurently?

Thanks

Tim