The reason that it doesn't work is because the installation guide is written incorrectly.
If you type the 2 lines that I describe in this thread:
Then, your dovecot will work, hence allowing you to sign into your email account using Squirrelmail (assuming you have made a valid email address).
Also note that the installation guide incorrectly shows how to setup Courier in the pictures for squirrelmail, but you really should be selecting the dovecot option in squirrelmail.
Also note that if you are following this installation guide, there will be numerous other things that don't work in your 'perfect server.'
Here are some links that tell how to fix some more of the errors that you will encounter as well:
And last, but the only one I can't figure out yet is this one:
Alas, also note that on step number 16 Install PureFTPd, you should install the openssl, then install the pure-ftpd. I have no idea why this certificate is installed in /etc/ssl/private/, but it will never be used by your server. Your server will use the one that is automatically created for you when you installed pure-ftpd. It is in the directory /etc/pki/pure-ftpd.