Not a technical issue but be sure to run such capturing/archiving past your legal folks also. In many jurisdictions employees, even using employer-supplied tools, do have some privacy expectations. Don't rely on some written policy in a HR manual to completely over-ride this, make sure you are actually complying with the specific requirements in your locale.
Back on the technical side consider also how you're going to store and search through this material, how you're going to identify what needs and need not be saved, how eventually the information is going to be destroyed. Keep in mind the first thing that happens now in a lawsuit is all relevant emails are subpoened. You'll need to be able to produce such in a timely manner, hopefully without putting undue burdon on your staff & systems. Incorporating such versatility in upfront will save you from a possibly painful, expensive, and potentially legally damaging retrievial/destruction project later.