add pkgsrc option for --enable-ip-based-virtual
This feature will will select the password file
and mail spool directory by the hostname that the user chooses to connect
to. (Of course, with this option, you will need separate IPs for each
virtual domain and the IP must resolve to the chosen host name.)
If the user enters the domain name with his username,
it will override the IP-based option.

other options:

VIRTUAL_UID is the UID of the owner (real Unix account) of the
  virtual mailboxes. 

VIRTUAL_MAILPATH
/var/spool/virtual

VIRTUAL_PASSWORDS_PATH
$SYSCONFDIR/virtual or /vm-pop3d

provide a pam file that has correct paths and a MESSAGE.PAM

add rc.d (rc.subr) script