My boss cites email to him as "Private communication". I want to cite email from a subject matter expert to a number of individuals (of which I was not one in the original message). It is not private nor personal because it involved 3-4 recipients. And it is wasn't initially sent to me. How does one cite this?
"Private" doesn't mean just one recipient; it just means "not public". When you throw a by-invitation party in your home it's a private affair even if there are 50 people there. Email is the same way.
The bigger problem here is that the person you're citing didn't write directly to you. So if you cite "Harry Q. Bovik, private communication" and somebody asks him about it, he's likely to say "huh? I've never heard of user36800".
If it's feasible, your best course of action would be to write to the expert and ask if you can cite him. If so, then you now have your private communication and can proceed.
If that's not practical, the most honest way to do it would be something like "Harry Q. Bovik as reported by Joe Shmoe, private communication". I've seen that sort of citation in less-formal writing; I don't think I've ever seen it in a journal article or book, though I rarely see private communication there to begin with.