the send tag doesn't display any error, if no correct email adress was set or add. it also doesn't display any error if the mail wasn't able to send.
If you use the error tag as the example shows, it will print out all email addresses that are not proper email addresses i.e. <characters>@<characters>.<characters> (I'm not sure exactly how it works but it is similar to this). Also since the mail is sent in the background as a thread, it will not print anything to the user. However it will log the fact that it was unable to send the email using the ServletContext.log() method. And depending on what is configured an email message should be sent to the from address telling the sender that the message was undeliverable.