"locked into a single kind of infrastructure" What does that mean? A mail server can be a small perl script running on an embedded computer, or it can be a hun

"locked into a single kind of infrastructure" What does that mean? A mail server can be a small perl script running on an embedded computer, or it can be a hundred front-end SMTP servers using a database as a back end datastore, there's no real infrastructure limit other than being able to make and receive connections on TCP port 25.