GMS can easily integrate in to an incumbent messaging infrastructure. How it does so depends on what role you would like GMS to serve.
GMS can be located in front of Sendmail, Exchange or other messaging servers in order to provide boundary protection. This allows internal systems to be totally isolated fron the Internet and for all mail to be virus scanned, checked for banned content, banned attachments, spam, and more at the point of entry to yoru network.
GMS can also be located anywhere within an existing messaging environment in order to provide additional services. These could be additional departmental email accounts, or simply additional services not already provided by the incumbent systems such as WebMail, Instant Messaging or list services including providing onward delivery only.