NameVirtualHost *:80

<VirtualHost *:80>
  ServerName www.foo.com
  DocumentRoot /var/www/www.foo.com/htdocs
</VirtualHost>

<VirtualHost *:80>
  ServerName foo.com
  DocumentRoot /var/www/www.foo.com/htdocs
</VirtualHost>

<VirtualHost *:80>
  ServerName mail.foo.com
  DocumentRoot /var/www/mail.foo.com/htdocs
</VirtualHost>