Mail in a Box
The guide on Mail in a Box's website states that it must be installed on a fresh machine.
Can I modify my box after / use my box for something else too?(Advanced.)
No. Mail-in-a-Box must be installed on a fresh machine that will be dedicated to Mail-in-a-Box, and you cannot modify the box after installation (configuration changes will get overwritten by the box’s self-management). If you are looking for something more advanced, try iRedMail, Sovereign, or Modoboa.