users / mailboxes etc

[...:::SHADI:::...]

hello!
  before deciding to install imp i was wondering how it manages users and 
mailboxes. do the users have to be (system) registered users or does imp
manage the virtual mail adresses with sendmail by its own?
  sadly i didn't find any information regarding this topic on the web
except that there is obviously a way to let imp "learn new users" (or
not).
  could anyone please point me to the docs or describe how imp handles
users and mailboxes?

regards,
  shadi