Retaining Returned Mail

We are in the process of re-mapping several of our accounts and we will be 'forcing' (for lack of a better word) certain customers into a new account type that may have different fees or terms. The question we have is regarding the 'returned mail' that we received that was the notification of the change itself. The mailing included disclosures, fee schedules and time tables for of the changes. Do we need to retain each peice of 'returned' mail, can we keep a list or can we just keep the associated envelope?