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I think we can give Glenn a pass here... Oftentimes, he'll use a few sentences or even a paragraph to elucidate just why he credits a particular source as trustworthy. In this case, I think the text of the emails presented, echoing NBC/McCaffrey/Weinerboy's talking points speak for themselves, even if the source has to remain a secret for the purpose of job security. Certainly a different case from Williams asserting trust in his consistent go-to war expert based on the shiny metal on his chest, and the fact that they like each other and hobnobbed on Pentagon flights to Iraq.
They're completely different. I'm not vouching for this person's general trustworthiness. If an issue were raised about his/her integrity, credibility, etc., I wouldn't be rely merely on their word and announce that all questions resolved to my satisfaction because I "trust" the person.
When I say they're a "trustworthy" source, I mean strictly for the limited purposes I'm using them here -- to believe that these emails are authentic. Why did I reach that conclusion? Because I know how s/he got the emails and -- most of all -- because the person has a public reputation which would be severely harmed if s/he fed me fake documents (which would obviously then lead me to reveal the who did it, etc.).
It's the person's self-interest and proximity to these documents which leads me to trust him/her for this limited purpose.