First, take a look at
http://www.e-smith.org/docs/howto/emailtransition-howto.htmlThis will be a good guide as to how to change over to SME for e-mail handling. As for handling mail for your M$ Exchange server, I *beleive* (and I hope someone will back me up here) the best solution is to create a backup MX record pointing to your SME server, and then create a template fragment (e.g. /etc/e-smith/templates/var/spool/smtpd/etc/smtpd_check_rules/21AllowExchange) containing the following, assuming the domain that exchange is handling is foo.com...
# Relay mail for the exchange server
allow:ALL:ALL:*.foo.com *@foo.com
Once this file has been created, run /sbin/e-smith/expand-template /var/spool/smtpd/etc/smtpd_check_rules and restart qmail using service qmail restart.
This *should* allow SME to relay all mail for foo.com. I caution that I have not tried this myself, and cannot guarentee that it will not cause SME to eat all mail it receives... (i.e. use at your own risk, unless someone else confirms that I've got it right)