Big difference between send_request_date and sent_date in sysmail_sentitems after running sp_send_dbmail, A send DB mail process was executed on Friday: 


placed on the mail queue at 2016-03-04 12:21:22.937, 
it wasn't sent until 2016-03-04 16:00:01.000, 1311 seconds later. 


Usually DATEDIFF(SECOND,send_request_date,sent_date) is at most a few seconds. What could have caused this massive time gap? 

Diagnosed via

SELECT *, DATEDIFF(SECOND,send_request_date,sent_date) 
FROM msdb.dbo.sysmail_sentitems;
Network Communication or a problem with the mailserver would be my guess. Check the log for the Mail server during this period. Chanses are you'll find it was unavailiable.
Regards Marten