I'll leave the software up to you.

I include Squirrelmail because it was on the qmailrocks guide and roundcube which is one of the most used (and I found it quite good)

For enabling HTTPS (which is quite recommendable) you have to do the following:

echo "NameVirtualHost *:443" >> /etc/apache2/ports.conf

a2enmod ssl

Then your virtual host file should look something like this

<VirtualHost *:443>
    ServerAdmin noc@example.org
    ServerName webmail.example.org
    DocumentRoot /var/www/squirrelmail
    ErrorLog /var/log/apache2/errores_squirrel
    LogLevel warn
<IfModule mod_ssl.c>
    SSLEngine on
    SSLCertificateFile    /etc/ssl/certs/ssl-cert-snakeoil.pem
    SSLCertificateKeyFile /etc/ssl/private/ssl-cert-snakeoil.key
    <FilesMatch "\.(cgi|shtml|phtml|php)$">
        SSLOptions +StdEnvVars
    <Directory /usr/lib/cgi-bin>
        SSLOptions +StdEnvVars
    BrowserMatch "MSIE [2-6]" \
        nokeepalive ssl-unclean-shutdown \
        downgrade-1.0 force-response-1.0
    BrowserMatch "MSIE [17-9]" ssl-unclean-shutdown