Why does "mailer dameon" keep e-mailing me (even though i sent the e-mail to the right address)?

i have to e-mail a guy for work purposes and he gave me his e-mail twice and i wrote it down correctly. however, both times i e-mailed him, i got an e-mail from "mailer daemon" saying the e-mail account was unavailable. the guy gave me his e-mail address five days ago. i don't know how to reach him otherwise. any help would be appreciated!