Thanks, popowich. I was leaning toward mbox but wasn't 100% sure. And thanks for the welcome. :D
On a side note, seeing you're from Rochester... I was born and raised in Elmira; my dad and youngest brother now live in Canandaigua. Small world!
I'd like to archive most of the past 15 years worth of business related email messages. Over the years I have migrated them between various programs, and each time was tricky due to different file formats that these programs used.
I'm currently using The Bat!, and it gives me the following...