Suppose that you have an encrypted message and a decrypted message (just one). M (the public key) and k (the exponent you raise each number to) are public. Does having one copy both version of a message give you enough information to decrypt other messages? I think the question boils down to finding the value of the euler phi of M which is not easy if you don't know its factorisation. I don't think there is a way to decrypt other messages but I'm not sure and am thankful for any help in advance.
Take any message you like. Encrypt it. You now have in your possession both an encrypted message, and the decrypted message. Does that help you to decrypt other messages?