Actually it’s very simple.
All you need is edit your DNS records to make sure your SBS external IP has the lowest preference.
Say your old POP3 preference 10 and your SBS is 11 or maybe 20, or perhaps you’ve not added it at all.
Simply edit your external DNS records to something like SBS external IP 10, POP3 server 20.
That’s it, all e-mails get delivered to your SBS.
One more thing though, does your SBS successfully send e-mail.
Just wanted to be sure that your SMTP connector is up and running. If it does, everything will work perfectly.