Hi sigmoun,
Look on your SPAM folder of your e-mail account. In monday release you'll be able to activate users from admin panel.
Anyway, if it's not in your SPAM folder, it should be an error in your email-server configuration. Since your friend received the email correctly, it should be a temporary error. If the problem persists, please, tell us your configuration to debug it.
Tell us if it's solved or not. Thanks!