Dividing indexes by year and month?

I have a large (over 1GB) mail file that I want to make accessible via the web. 
There are over 700,000 messages. I ran mhonarc and I got index files that were 
over 150mb each, and a directory with over 700,000 html files. I was wondering 
if there was a way to have it divide the files in directories by year and 
month, and similarly divide the index files up so they're not so large. Is 
there a way to set the maximum number of messages in an index file, and then 
have it paginate to another file?

Thanks,

Philip