Mail servers are state machines. The SMTP protocol is all about state. In a way any web app with a login is also a state machine - there's the logged in state

Mail servers are state machines. The SMTP protocol is all about state. In a way any web app with a login is also a state machine - there's the logged in state, and the not logged in state. erlang has a very effective state machine library(behavior). But the downside is that it's erlang. http://www.erlang.org/doc/design_principles/fsm.html http://www.erlang.org/doc/design_principles/fsm.html