We always assume that mail sends successfully, so when it fails, we give the user confusing messages.
drupal_mail() returns the $message with the send results in $message['result'], we should check this before displaying any message that says something like "Your mail has been sent". The assumption is that the underlying mail system displays it's own error message on failure. So, on failure, the underlying mail system might say "Your message has NOT been sent", and then Drupal comes over that and says "Your message has been sent." ... bad, confusing.
Would be nice to get some fix in quickly ... and then have further discussion about what is the right way of handling these failures.