My suggestion would be to use "milter" to scan/copy each message to an archive folder, or simply send it to an archival mailbox. Doing that, you could capture all inbound and outbound mail, and even compress/database it if you wish.
Totally none of my business but I guess thats the point. Why do you want to copy all of your staff emails? Honestly, no offense meant but don't you have better things to do with your time or are you worried about something specific?