I found these instructions lacking a little. If you create a new user, you have to take an extra step. You need to create a maildir for the user before you setup the user in the database.

You need to create a folder for your users (example for bofh@YOURVIRTUALDOMAIN.com):

mkdir /home/vmail/YOURVIRTUALDOMAIN.com
mkdir /home/vmail/YOURVIRTUALDOMAIN.com/bofh
cd /home/vmail/YOURVIRTUALDOMAIN.com/bofh
maildirmake Maildir
chown -R vmail:vmail /home/vmail/YOURVIRTUALDOMAIN.com/*


Now you're ready to insert the user into the SQL database. This avoids having mail accepted without a place to put it. After you insert the user into the database, you should be able to login via IMAP and see an empty directory. You should also now be able to send mail!