Exception Message: Cannot send mails to mail server. (The specified string is not in the form required for a subject.). )

hi,
I had check some mail are properly send but 2 mail are not send.
I had see this error in select * from msdb.dbo.sysmail_log.
The message show:-
"The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 10 (2012-08-16T03:00:02). Exception Message: Cannot send mails to mail server. (The specified string is not in the form required for a subject.). )"
 
Please some guide me...