sql server在触发器中作为权限错误执行。

I have a trigger where I want to send out an email on updates to a row in a table SalesClosing. Now the user (dbuser) who execute the trigger has very limited permissions. So I want the trigger to execute as dbmailuser. A user who has rights to send out email. I tested that dbmailuser can execute sp_send_dbmail when logged in as that user. When the trigger fires though, I get an error can't execute sp_send_dbmail. So I logged in as dbuser , ran EXECUTE AS LOGIN = 'dbmailuser'; and was able to execute sp_send_dbmail. Now why can't I do that in a trigger. I'm using sql server 2008. Below is the trigger ddl.I have a trigger where I want to send out an em