I was thinking about implementing a posterous-like email system for calendaring, and was wondering how they authenticated the emails. I recall them getting "hacked" around launch and there being some TC article about how they responded swiftly by adding new security measures.

Just registering the "usual" smtp sender / relay and prompting the user before posting something from a different spot could help. I don't know enough about MX records yet, but matching up the domain and sending IP could be another good measure. How else can this be improved?