[yehosef]

>What I don't understand is why they chose to redact the username part of the address for the fb.com emails, but chose to instead redact the host part of the address for the external recipients.

I think they understood the others worked at facebook, what needed to be redacted to protect their privacy is the name. For the external people, we know what their names are (some are govt officials which is why this is being FOIA'd) but not the domains, it would seem.