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

[Kendall Shaw]

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