By default all outbound traffic is accepted, so I can't think of a reason why it's not accessible.
One solution is to test using another connection that does not use the SME Server and attempt to connect. If it works and you can access the site using another connection then there must be some rulesets in your firewall to disallow accessing such a high port number. There could also be other reasons why you can't access it. I know that webmin has the feature to allow only certain IP address to be able to access it which could be why you can't access it. If you know the person who is running the webmin app then contact them and ask them to add a trusted referer entry into the webmin configuration with your IP address added in there.
If there are still further problems with accessing the webmin app then you need to closely inspect what ports the server that is running webmin on is allowing incoming connections on and also inspect the trusted referers list closely.
Hope this helps....