Account suspended? Why?

I started to use my e-mail to save some of the works I had done. Then suddenly I get a failure notice from MAILER-DAEMON with a message saying my account has been suspended. So I got a new account and after awhile I am getting the messages again. Why is that? I always sign off of my e-mail after I use it and no one was looking over my shoulder as I typed the password. Why am I getting these failure notices?