Check If sp_send_dbmail Was Successful

sysmail_faileditems will only get you the list of failed emails. If you need to see a list of successfull emails you need to use sysmail_mailitems.

Use the below query to get details for all emails sent the same date:

SELECT * FROM msdb..sysmail_mailitems WHERE sent_date > DATEADD(DAY, -1,GETDATE())

And here is the complete query to get all the failed emails from the past 24 hours:

SELECT items.subject ,
       items.recipients ,
       items.copy_recipients ,
       items.blind_copy_recipients ,
       items.last_mod_date ,
       l.description
FROM   msdb.dbo.sysmail_faileditems AS items
       LEFT OUTER JOIN msdb.dbo.sysmail_event_log AS l 
                    ON items.mailitem_id = l.mailitem_id
WHERE  items.last_mod_date > DATEADD(DAY, -1,GETDATE())