Impersonation Through EXECUTE AS Failing

Hello all-
Before I go any further, I have followed http://msdn.microsoft.com/en-us/library/ms188304.aspx as best possible. I am attempting to send mail through a DML trigger. We'll call the database 'DB', and it is owned by a domain account named 'DOMAINAcct'. The trigger simply blocks any CUD operations on a table which we'll call 'Tbl', and sends an email. Hence, it looks something like...

CREATE TRIGGER [dbo].[TR_Tbl_BlockChanges]
ON [dbo].[Tbl]
WITH EXECUTE AS OWNER
INSTEAD OF INSERT,DELETE,UPDATE
AS
EXEC [msdb].[dbo].[sp_send_dbmail] @profile_name = 'AcctMail', @recipients = 'foo@bar.com', @subject = N'CUD operations not allowed on Tbl', @body = N'Blocked'

AcctMail is a valid profile and operates correctly. I have created the DOMAINAcct user in msdb, given it the AUTHENTICATE permission, and added it to the DatabaseMailUserRole. When the trigger fires, according to the article, the security context should switch to dbo (DOMAINAcct), then be successful when attempting to execute the msdb sproc. Instead I get the usual:
Msg 229, Level 14, State 5, Procedure sp_send_dbmail, Line 1
The EXECUTE permission was denied on the object 'sp_send_dbmail', database 'msdb', schema 'dbo'.

Thoughts?