OfficeMail from Burrotech looks to be an ideal way to provide email for all users on a small network, without each of them needing an Internet connection. There’s a free and shareware version available.

‘In other words it gives everyone on a LAN spam- and virus-free email – [sic] both to everyone else on the LAN and to the outside world. Install Office Mail on one PC (it doesn’t even have to be a dedicated PC – see the FAQ page) and everyone on the network can use it to send and receive emails from their PCs.’