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Some clients report to us that our site is not accessible through their internet connection. We suspect our site is wrongfully blocked by some security software/firewall/public blacklist.

How can we verify that, other than trying them one by one? There are so many security software out there that it is not practical...


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Over IPv4 oder IPv6? By "Site" you mean Website (http) or other services? – MacLemon Apr 7 '10 at 17:57
verified with client, they block sites with https webmail login, wtf. :) – Henry Apr 9 '10 at 20:54
Henry, did you ever get the solution to your problem? If so, you can mark one of the answers belong as the answer to your question so that future readers will know what helped. – Wesley Apr 23 '10 at 3:43

The first thing you need to do is try to figure out what software your clients are using that's blocking your site. (Or maybe it's their ISPs that are using something.) Yes, there are lots of possibilities, but the first one to start with is probably Websense. Even if Websense isn't responsible for your clients' problems, it's worth seeing how your site is categorized.

Websense has a "site lookup" that you can use to see how they categorize your site, but you have to set up an account and log in to access it:

From Websense's contact page you can (without registering), check how they categorize your site with SurfControl (another filtering product that Websense bought out) and you can request that they re-categorize your site if it's wrong:

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I use for most of my mail trouble shooting. After you lookup your domain, there is an option for blacklist at the right of the result that will check a plethora of sites that blacklist domains. You can also try The middle column has Spam DB Lookup. As well as a lot of other tools to make sure your domain might be pointing to the correct IP address and networking troubleshooting.

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Doesn't this only check if our MX mail server is blacklisted? – Henry Apr 7 '10 at 2:46
I think you're right. You can change it to check so it isn't just checking the mail server's IP address. I've never heard of a web site being blacklisted unless maybe they are using the same firewall where your site might have been added to a filtered list. Are they getting "page can't be displayed" or some other message? – Nixphoe Apr 7 '10 at 2:57
MXToolbox is only for measuring SPAM metrics, not harmful websites. – Wesley Apr 7 '10 at 5:43

It could also be a routing problem. Can they ping your site by domain name and also IP address? Do a tracert or pathping to see if there's a breakdown in their local routing system to your website (maybe their ISP is flaky?). Are they using OpenDNS perchance? OpenDNS's community of domain taggers has been known to get a few things wrong here and there. Go to OpenDNS's domain taggins section and search to see what's being said about your domain:

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