It seems like this has already been answered in a previous thread: https://wurstforum.net/d/63
In short, the issue is that cracked accounts can't access premium or "online-mode" servers as they only allow legitimate accounts bought from Minecraft.net. On the other hand, cracked servers allow these accounts but come with their own limitations. The "authentication servers are currently not reachable" message suggests that you're trying to join a premium server and you'll need a valid Minecraft account to do so.
Hope this clears things up! Let me know if you have any other questions.