How can I check for errors, I did everything you listed and it still does not work.
I created a DNS A record for webmail.example.com with internal IP.
then I did the Virtual host record on the mailserver (hostname mail.example.com)
then I followed the rest of steps for squirrelmail except replacing with roundcube.
Externally it still does not work.
I also realized that I made 2 modifications to roundcube.. first was
which I added
The second I created through webmin a virtual host on the apache server for the directory of /var/www/roundcube