Let me get this strait, you want your apache test page to be visible from the outside, when someone enters http://your-chose.dyndns.org into their browser right ?
I'm assuming you know how to maintain the IP info on dynDNS, so it will allways have your appropriate IP info as your-chose.dyndns.org, else focus on the DDNS Client mentioned in Dynamic DNS
Once you get that sortet, it's a matter of focusing on Named Virtual Hosting in the The Apache Web server howto.
For a quick example, you can create a file in /etc/apache2/sites-available/ called your-chosen.dyndns.org which holds:
And then link to it from /etc/apache2/sites-enabled/ like so:
ErrorDocument 404 /my-error-page.html
CustomLog /var/log/apache2/my-dyndns.org-access.log combined
Options Indexes FollowSymLinks MultiViews
Allow from all
Then restart apache, to see if it will be visible by http://your-chosen.dyndns.org. Given that you've created the /var/www/my-dyndns directory and has a valid index file there...
> ln -sf /etc/apache2/sites-available/your-chose.dyndns.org /etc/apache2/sites-enabled/your-chose.dyndns.org
As for postfix, you need to configure it, so it will accept mails for the your-chosen.dyndns.org domain, so for that focus on the Postfix Virtual domain hosting documentation, and perhaps the Postfix on hosts without a real Internet hostname portion, else good luck and hopefully it will work out.