Let's be clear
As a system administrator for a major ISP, I can assure you that the moment you decide to use mail services other than your own private mail server, then mail tends to be "backed up" regularly ( for an unspecified, but lengthy intervals ) -- including "deleted" mails. Moreover, if a government agency tips off an ISP that they need to target a user, the entire host of storage devices associated with that account is vaulted and shipped for deep forensic recovery ( of any deleted mail spools too ). In addition, deep packet analyzer tools ( such as "carnivore") are linked with the interface serving the subnet which hosts the account.
Bottom Line: Company or Privately hosted doesn't make a bit of a difference. Short of physically destroying equipment there is no guarantee that any evidence can be purged.
