I got this on a box the other day: postfix/postdrop[21416]: warning: mail_queue_enter: create file maildrop/465240.21416: No space left on device My question

I got this on a box the other day: postfix/postdrop[21416]: warning: mail_queue_enter: create file maildrop/465240.21416: No space left on device My question is, if that is a warning, what's an error?