Which mail server are you running? Is it unix or windows based?
Do you force all of your employees to send email through your server?
If they are sending through their ISP smtp relays from home computers you probably can't store a copy of them.
You can make a company policy that says they are required to send mails From: your company domain only using company mail servers.
You can also try to back that up with SPF policy that tells other mail servers to reject email From: your domain when it's not sent from your company mail servers.
It's not perfect, but it's a step in the right direction.
Back to my original questions, depending on what mail server you are running, you may be able to configure it, or add new functionality, that allows you to store a copy of all outgoing emails.