I just set up a message to send, sent it to the delivery queue, and then went to the "Deliveries" tab and can't find the message. Then when I go back and look at the message, it shows as Approved but never sent, what happened?
Email sends that fail in the audience calculation stage do not jump to the top of the Delivery List like newly queued messages do. Because they never actually make it to the delivery queue, they will have a status of "Rebuild Error" and may jump to the bottom of the list. Try searching for the name of the message in the search bar or organizing by "Status" to find messages with a rebuild error status. You will see the error regarding why the audience calculation failed in red underneath the message name.
Note: This problem quite often is caused by the target audience containing a query group that cannot rebuilt (often because there is some object referenced in the query, like another group, that no longer exists). If you get an error message of the form "unable to rebuild group inline (queryID = XXXX) group = XXXXXXX", go to the group information page for that group and try to manually rebuild it by clicking the "Rebuild Group" button. A red error message should pop up that will explain what is wrong with the query that is preventing it from running.