Recommendations for email packages

I want to provide some type of email type functionality 开发者_运维知识库to the users of my system. Basically their own web-client inbox and the ability to do peer-to-peer messaging (emailing). I dont see this doing a lot of interaction with outside mail providers, mostly just internal users of the system.

Can someone give me pointers as to what open source packages to start off with? I am not quite sure whether to start with social networking type packages or email packages.

Thanks.

I strongly recommend Proteus for your needs, since you would need two types of messaging integrated (email and IM).