Silent mode in the heuristics mode gives three options in the Internet mail provider (as I believe does the outlook/exchange provider).
However, if you are using Outlook and not OE then the outlook/exchange provider is what you should use (otherwise you are using the wrong provider and may be why the silent mode doesn't appear to be silent or there are no options set).
The Internet Mail provider is for non outlook/exchange email programs, so it's of no use to you unless you are using OE or another non Outlook program.
What mail program are you using, I assume it's not OE?
I'm not sure of the exact options available in silent mode in the outlook provider (I had thought they would be the same)
HTH David