Database mail configured and working successfully but log details showing error

Dear Experts,

Please help me out this issue.

I have configured the database mail setup in sql server 2012 evaluation version but facing following issue.

1- I have tested many mails and it working fine and mail is getting delivered on given Email id but when i see the mail log (sysmail_mailitems)details its showing that mail is send successfully. but the history log file showing that mail could not delivered due to some issue.

 as you can see the attached screen for record no mailid 1040 status is 1 but still in  log history table it showing error.

2- the second issue i m sending a test mail and its delivered successfully but no any table data is getting changed but when i change and enable xp mail services then only table data is getting  change

SELECT TOP 1000 [mailitem_id] ,[profile_id],[recipients] ,[sent_status] FROM [msdb].[dbo].[sysmail_mailitems] where mailitem_id=1049

mailitem_idprofile_idrecipientssent_status
10496 nirbhay.singh1@qsp.net1

SELECT *  FROM [msdb].[dbo].[sysmail_log] where mailitem_id=1049

log_idevent_typelog_datedescriptionprocess_idmailitem_idaccount_idlast_mod_datelast_mod_user
10813 2016-04-28 14:11:10.913The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 6 (2016-04-28T14:11:10). Exception Message: Could not connect to mail server. (No such host is known).
)1648 1049NULL 2016-04-28 14:11:10.913sa

please suggest,

Thank in advance.