I can't really say that I have enjoyed email games in the past. Maybe because they often did not reflect the topic being studied or include tangible activities or clear instruction. Here are some tips to help you design effective email games.
Your email game should engage your participants in a meaningful discussion. For example an opportunity to problem solve or dicuss common issues relating to the topic being explored in their study.
Make sure your game allows the participant to bring their own wealth of knowledge into the game and provides an opportunity to reflect on discussions and findings.
Clearly set out the rules for the game. State when the game will start, how the game should be played, who can play, when can they play, how participants can score points and how the participants will know that the game is over. Define what will happen after the game is over.
Your game can be played using email alone or it could be played in collaboration with other technologies such as discussion forums. It is really important that the relationship is set out clearly in the initial game instructions so the participants know when to use the different technologies.
Another useful technique is to assign each person a role and a topic or challenge for exploration (eg: if your topic was to do with simple strategies to keep the national park tidy you could divide the roles into Park Ranger, Kiosk Manager, Tourist etc). The information can be sent directly to the facilitator for compilation and then later posted on a wiki or discussion forum or you could ask the participant to post their contributions directly during the game.
The design and communication strategies implemented must be appropriate to your group and study requirements. For more techniques and strategies visit http://community.flexiblelearning.net.au/. It is a great starting point for those of you who are new to e-learning.