What is the message stated in the [sysmail_event_log].[description] column if you execute the query below?
SELECT *
FROM msdb.dbo.sysmail_allitems
LEFT OUTER JOIN msdb.dbo.sysmail_event_log
ON sysmail_allitems.mailitem_id = sysmail_event_log.mailitem_id
ORDER BY
send_request_date desc