[wlug] Email training system

Does anyone know of a email server, preferably free, that can be used in a training environment? Current requirements are POP3 and SMTP access. All email is to be confined to a single domain (the classroom - not registered) and access to the (external) Internet should be blocked. It appears there could be a fair amount of work in setting up "standard"software for this limited task, but I would prefer something that most people would find simple to install to maintain. Michael