If you were to browse to http://ip_of_server/webmail
I'm going to assume that you get the squirrel mail interface.
If this is the case then there are a couple of very simple ways in which we can make a FQDN to point at the webmail.
The easiest and quickest way would be to add a server alias to the default config file in /etc/apache2/sites-available. E.g:
Then restart apache - /etc/init.d/apache2 restart
This will result in http://webmail.example.com/webmail
now showing the webmail interface (provided you configure DNS to point webmail.example.com to point to the server's IP address).
The other option is to create a new virtual host called webmail.example.com and point the document root to /var/www/webmail. That way http://webmail.example.com
will point directly at the webmail interface.
If you need any advice on configuring a new virtual host to perform this operation then let me know.