[Dovecot] auth both system and virtual users

[Gerry Reno]

Followup:
  The auth is working for system users but not for virtual users.  I have
put the virtual user, tuser, in the file /etc/imap.passwd in /etc/passwd
format using the example.  But I can never login with this user.
Here is /etc/imap.passwd:
tuser:{PLAIN}pass:65534:65534::/var/mail/u/tuser

Other questions:
  When I logged in the first time with system user, greno, it created the
following tree under /var/mail:
./r/greno/Maildir/new
./r/greno/Maildir/cur
./r/greno/Maildir/tmp

  This is great except that the top level directory, 'r', is owned by this
user alone with very tight permissions.  What will happen with next user
with last name beginning with 'r'?
  drwx------ 3 greno greno 4096 Oct 30 21:11 r
Is there a way in dovecot.conf to make this owned by dovecot or root and
more open permissions?
Or should I create whole alphabet directories, 'a','b','c'..., and assign
them to dovecot owner?