Try this: Open a command prompt on your computer by clicking on ‘Start’, ‘Run’, and typing ‘cmd’. If you have Windows 7, go to 'Start', 'Programs', 'Accessories', 'Command Prompt'. Type the following at prompt: ipconfig /flushdns. You should get the following message: Windows IP Configuration Successfully flushed the DNS Resolver Cache.
The ipconfig /flushdns command flushes your Windows DNS cache. The DNS is what translates things like www.empireminecraft.com to an IP address like 110.232.140.130. Windows saves some of these addresses in a cache and I'm guessing it may be the cause of your problem. If your DNS isn't working correctly, it's possible to get messages like in your screenshot: Unknown Host session.minecraft.net. Since I can log onto the servers and I don't see a bunch of other threads with people complaining, I think the problem is local to your PC or your ISP. If it it were your ISP, you wouldn't be able to get to this website so I'm suggesting you flush your local DNS cache. It won't hurt anything, but may fix the problem.