Hi Merv,
Most likely the email would have still sent to the other email address.
If the email sends and you later get back a bounce message, remove that email address from your list.
Removing the bad addresses from your list will help prevent your account from getting blocked by your email service.
If you get the error message while sending, such as a popup message from your mail server, then it's more likely no one else received the email. If this happens you should also still see the email in your outbox.