hi i have a situation working that might be similar, all the mail comes to the sme, then to those using exchange we push that users email to the exchange server. setting in server-manager, user, email delivery, forward email to address below
last time i checked exchange can't pull mail or use pop. it does get a bit dirty as you will have to have the domain names of the exchange box different to the sme but also able to use the same domain name. so each email account on the exchange box must use 2 domains, so local mail on the exchange works, but users on the same domain name but on the sme box gets their mail.
this xmas i have dumped the exchange box on a few of these sites and replaced them with sme. everything runs much sweeter, thrown out outlook and put in thunderbird using imap
bit complicated and messy but it does work
hope it helps
thanks
john