I wouldn't be surprised if there are hardcoded rules that guard senior Google execs against such inadvertent (or by design, take your pick) communications from hoi polloi that uses their products.
A whitelist wouldn't work, because these people need to be contacted from the outside, from people with gmail addresses (usually goes through a secretary though)
A dedicated (unfiltered) internal email sounds more likely, or just routing directly google.com to google.com addresses.
But then again this is google and maybe internally it's only Hangouts
