[Dovecot] multiple Authentication

[Marc Perkel]

I'm using the following authentication passing the full email address 
user at domain.com

auth default {
  mechanisms = plain
 
  # Linuxconf passwd-like file with specified location
  passdb passwd-file {
    args = /etc/vmail/shadow.%d
  }

  # Linuxconf passwd-like file with specified location
  userdb passwd-file {
    args = /etc/vmail/passwd.%d
  }

  user = root
}

But - what I want to do is if the user doesn't enter the domain part I 
want it to assume one specific domain. How would I do that?