well now i can connect to the server, but i still cant type commands (/help, /home, etc.), WTF???

When I try to connect to the minetown server, I get a message saying "server responded in with invalid key," or, "server outdated." What should I do? Also, when I connected to other servers in the past, I would try to type a command (/help. /home, etc.), nothing would happen and i would end up just opening my inventory (from pressing e).