You need to double quote everything because your query is inside a set of quotes. Example:

EXEC msdb.dbo.sp_send_dbmail

@profile_name = 'AdventureWorks2008R2 Administrator',

@recipients = 'danw@Adventure-Works.com',

@query = 'SELECT COUNT(*) FROM AdventureWorks2008R2.Production.WorkOrder

WHERE DueDate > ''2006-04-30''

AND DATEDIFF(dd, ''2006-04-30'', DueDate) < 2' ,