[Dovecot] why is there email in my mbox file?

Hi,

I've installed dovecot-imapd under ubuntu. My account and root are the only email accounts on my computer. I transitioned from using GNUS with fetchmail, exim and movemail, to gnus with exim, fetchmail and imap, so I've kept the mbox format email instead of changing to maildir. This works okay, except my mbox file is giant.

I expected dovecot to take the mail out of the mbox file and put it somewhere. But, I see there's a big mbox file (/var/mail/me) that keeps growing. Is dovecot using that as the store of messages?

If I were to move to using maildir instead of mbox, what is going to happen to my existing mail?

Kendall