When posting a message on one of our forums, it is not unusual to get a pink box containing the message "Unable to send email. Contact the site administrator if the problem persists.”
This is a false alarm and you can ignore it. The email notification of your forum post does indeed get sent to everyone subscribed to the forum with a valid email address. (The message is generated whenever there are any invalid emails on the subscriber list - even if there is just one).
Unfortunately, we have not yet been able to fix this bug, but it is on our list.