All mail to and from your employees wil not leave the internal mailserver.
Internal mail is possible, even without internet access.
All mail will traverse the mailserver. This is the only way that mail can be sent.
The mail search options are really fast and will search the complete mail (headers and body).
Secure Webmail access.
Ensure traceability of mail: Log all incoming and outgoing mail.
Backup mail to external storage, completely automatic, no actions required.
Move mail the mailarchive after a certain period of time.