Run these from APEX SQL Workshop:
SELECT * from APEX_MAIL_LOG ORDER BY LAST_UPDATED_ON DESC;
The next one will normally be empty, unless just prior to a queue purge (every 15 min?) or an error happened, then it will be retried 10X before finally being purged:
SELECT * from APEX_MAIL_QUEUE ORDER BY LAST_UPDATED_ON DESC;
simple. 