Error 554 - Poor reputation

Error 554 - Poor reputation

Error, ERROR_CODE :554, ERROR_CODE :ausgabe.rsccd.org 
            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. 

This is the error I receive when trying to send e-mail to a professor at my college. E-mail is the primary way I turn in homework and classwork, so this is very serious to me. I've only just started getting these errors today, as I've e-mailed him and other professors using the same e-mail provider (the school) for almost two years now. I don't receive or send many e-mails, so I'm not entirely certain why I'm getting this error (which I assume means I'm blacklisted somewhere). The e-mail I'm trying to send from is shawn@shawnyall.us, and I'm trying to send to a .edu address, the mailserver of which is listed in the error code.