What does this mean: 550 Requested action not taken: mailbox unavailable [RCPT_TO]?

i tried sending an e-mail last night to a hotmail account that i know exists and i got the following mailer-daemon message in return: 550 Requested action not taken: mailbox unavailable [RCPT_TO]. i know the e-mail account is in use so why did i get the e-mail from mailer-daemon saying it wouldn't send? is there any chance the e-mail still went through? thanks!