The basic problem with trying to do records management in this fashion is that it relies too much on the end user to determine what information needs to be kept and for how long. No matter what mail system you use, expecting users to manage the retainment of correspondence that could have legal consequences is not exactly what I would call a bullet proof solution. After a long investigation, the only solution that I came up with is to make a copy of every piece of correspondence that is sent in the organization. Allowing users to classify that information after it's been captured is always an option, but making them determine what should be saved is a recipe for disaster.
Aside from that, how do you envision handling emails that are encrypted? Also, since you are planning on moving it Sharepoint, how do you handle data integrity of the underlying meta data?
Sean---