Following on from part 2 of this guide:
Install the maildirmake utility, which is part of the maildrop package.
apt-get install maildrop
Create a group for the virtual mail server, then create a user and add the user to the group:
useradd -g vmail vmail
Take a look in your /etc/group file and note the GID number of the vmail group. On fresh server installs, this will probably be 1001. Then take a look in your /etc/passwd file and make a note of the UID of the vmail user.
Now install an IMAP server – we’ll use courier-imap.
apt-get install courier-imap