**UPDATE **
I checked the PRD server where the mails are still working and ran
SELECT * FROM sys.service_queues
execute_as_principal_id = 1 on sp_ExternalMailQueueListener and sp_sysmail_activate
On the QAS server
execute_as_principal_id = -2
and checked http://msdn.microsoft.com/en-us/library/ms187795.aspx
ID of the EXECUTE AS database principal.
NULL by default or if EXECUTE AS CALLER.
ID of the specified principal if EXECUTE AS SELF EXECUTE AS <principal>.
-2 = EXECUTE AS OWNER.
I think this is the issue as this is the difference between the 2 servers.
ALTER QUEUE [InternalMailQueue] WITH ACTIVATION (
STATUS = ON,
PROCEDURE_NAME = [dbo].[sp_ExternalMailQueueListener],
MAX_QUEUE_READERS = 1,
EXECUTE AS OWNER);
ALTER QUEUE [ExternalMailQueue] WITH ACTIVATION (
STATUS = ON,
PROCEDURE_NAME = [dbo].[sp_sysmail_activate],
MAX_QUEUE_READERS = 1,
EXECUTE AS OWNER)
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