Mojang is blocking certain servers as of 1.9.3 via a online list containing the sha1 of the hostname the client wants to connect to.
This repository automatically tracks the changes to this list, every five minutes.
https://www.reddit.com/r/Minecraft/comments/4h3c6u/mojang_is_blocking_certain_servers_as_of_193_r2/
https://www.reddit.com/r/admincraft/comments/4h3d99/incoming_session_server_blacklist_193/