We all know how email works. You email a colleague to test out an idea or a proposal and if his response is useful you forward it to a third person with a comment. Or you forward a sent email to another colleague with a comment such as 'just seeing how Bob responds to this'. We all do it every day. In such cases party B's emails never contain party C's email addy - but this is hardly proof that party C didn't know anything about them.
Now suppose you're Gus O'Donnell and you've seen the Downing Street email logs that show that party A had forwarded such emails to party C within Downing Street. But party C had taken a public line saying he'd never seen such emails, and engaged m'learned friends to frighten the media if they suggest differently. What would you say in an official statement?
"Tom Watson has made his own position clear" would just about cover it, I reckon.