"Undelivered mail returned to sender" error

I have been trying to contact a professor at my college but keep receiving this error:

" Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means. "

I' m not sure what this means, but I'm unable to send e-mails to him as of today. I've sent and received email from him before as e-mail is the primary way through which I turn in homework and classwork. I don't send or receive very many e-mails on this account, so I'm not sure what might have happened to trigger this response. (Which I'm assuming means my e-mail address is blacklisted.)

The e-mail with this problem is shawn@shawnyall.us