SQL notification odd user

Hi, I've setup the SQL Server server module, using a specific user login (who I know has permissions, as tested and used when checking the query later).

I've then setup a notification using it, based on a query.   However when that runs I get told that "NT AUTHORITY\SYSTEM" can't access the database.  This doesn't surprise me, but I don't understand why it's trying to connect as that, and not the user I set up.

Any ideas?

Thanks