I once took a call with an ex-NSA guy who was a CEO selling email security software- your MX would point to them, they'd scan your incoming email for exploits, then deliver it on to you. When I spoke to the guy, I expressed my concern, having working for a large multinational corporate whose fiber optic lines were tapped by UK intelligence to avoid NSA laws against spying on americans, that if I couldn't inspect his software, I couldn't feel confident of the security and integrity of the scanning system.
I said, in good faith, that I would consider his product if I could inspect the running system and the code.
He said several things:
1) the NSA never did anything illegal
2) the software was too large to audit
3) it was an insult to his employment in the NSA that I was even asking these questions.
Then he hung up.