Geez, I sent an email to a friend tonight and................?

it could not be delivered. The funny thing is that we had been emailing back & forth just about an hour before I sent this last email. It got returned, so I resent it. It got returned again. Said something about a failure notice. So, I composed a new email and tried to send it, but it got returned, also. Thinking there was something wrong with my email, I composed another email to someone else and it went through. Why would this happen? Does anyone have a clue? I just sent two more emails to two other people and they both went through. I just tried to send another email to my undeliverable friend and the same thing happened again................said it was undeliverable. I am perplexed.