All you need to do is to enable it in the server-manager, just click on Email under the Configuration heading on the left hand side of the server-manager web page. Then forward the appropriate ports if you have a router between the server & the Internet. Remember that SME use secure POP, which uses port 995. Outlook, Outlook Express, Thunderbird & Apple Mail all support secure POP, so I imagine most other email clients do.
HTH
Peter