Would not allow to play a Minecraft server.

Dear Mojang,

When I tried to play Cubecraft ( A Minecraft server ) It wouldn't allow me to play the game.
It kept saying '' Not Authentic to Minecraft.net. '' Then, I tried to play Guncolony, it said the same thing again. Then I tried to play Hypixel, it STILL wouldn't work. I hope you can fix this issue.

                                                    Thank you. 😃