[Dovecot] basic understanding of imap in dovecot

[dovecot at paz.bz]

I have always used a pop3 daemon, and switching to dovecot has been my
introduction to imap.
Imap is foreign to me.

I am wondering if I am a little off-track here, and would like some
clarification.

My MTA (exim) delivers email to:   /mail/user
dovecot, sucks that mail from /mail/user TO
/home/user/mail/<filenamesoffolders>
dovecot IMAP further uses /home/user/mail/.imap/folders/{.index /
.index.cache / .index.log}

the mail folder which exim uses doesn't seem to get emptied by dovecot
when it sucks the email
into /home/user/mail/<filenamesoffolders> when using IMAP.
If POP3 is used via dovecot, then it appears that the interaction is
directly to the original
exim folder and that it does indeed empty (or at least it has the "FOLDER
INTERNAL DATA" entry).

I suppose my question is, is there a way to have dovecot empty/parse the
original exim mailbox
so that the customer email isn't sitting in two spots, taking up double
the space. This would
be in the IMAP environment only, as POP3 seems to be fine.

Any online references available to help a newbee understand IMAP?

Thanks!