Recently CraftBukkit has taken measures to ensure the safety of Multiplayer Servers by adding a feature that will block unsafe IPs. This feature utilizes an independent company that sole purpose is to monitor and track IPs. If you try and connect and you receive a message that "Your IP has been flagged as unsafe by http://www.spamhaus.org/" you will not be able to connect to the server until you get it unflagged.
If you have this issue you will need to go to http://www.spamhaus.org/lookup/ and follow the steps to get your IP removed from the blacklist.
MutinyCraft has no way to remove your IP from this database and it is up to you to follow the steps in order to remove your IP from their site. I do have the power to turn this feature off and I have decided that for the safety of the server I will leave it on. I know of only 2 people who this has directly affected and I am confident if they take the proper steps to contact the company mentioned that their IP will be removed from the blacklist.
If you have any questions feel free to message me.