Just a quick point:  although not the case with IMAP, only one user can
access a POP3 mailbox at a time.  There is a great probability that
m0n0wall/network might hiccup, and lock the user's mailbox.  Or, the
customer just might happen to hit send/receive while m0n0 is logged in.

I have no need for it, but if it were me, I'd build a script to export
my mail users to a text file, and upload/scp to an ftp/https location. 
Then have m0n0 run a script to import said file into m0n0's local
database.

Just my two cents.

-- 
Justin Ellison <justin at techadvise dot com>