Hello,
The feature is available but requires the enterprise license:
http://help.smartertools.com/smartermail/v4/Default.aspx?qq=%2fSmarterMail%2fv4%2fTopics%2fSystemAdmin%2fConfig%2fMessageArchiving.aspx
I have also seen third party companies that do the archiving in an exchange environment such as Global Relay:
http://www.globalrelay.com/
I am not sure if it requires the exchange environment though so it will require contacting them or possible someone else can reply that has used it in this kind of setup but I mentioned it because there are companies out there that will archive your email for you.
Matt Pellegrino
Support Engineer
mpellegrino @ hosting.com
www.Hosting.com