Not legal in sane jurisdictions. There is no guaranteed delivery of email. Important stuff still needs to be sent by confirmed fax, or snail mail with various options to confirm delivery to the recipient.
Where do you draw the "important"-line? Most my (important) bills come in my snail mail mailbox without any confirmation to the sender. If i don't pay them on time i get a delay fine. What's easier, pay the fine or try to prove in court that you never received the invoice? Or worse, explaining that you threw out all the mail you received during holidays?
Still, I agree, important messages through email should always be replied to to confirm that you've got them.
