You could try taking a look at the mail item views, they may uncover something:
SELECT TOP 10
f.mailitem_id,
f.subject,
f.body ,
f.send_request_date,
e.Description
from dbo.sysmail_faileditems f
LEFT JOIN dbo.sysmail_event_log e ON e.mailitem_id= f.mailitem_id AND e.event_type =...