I hate to make things more complicated with more of my own speculation; but it may also be possible that American intelligence is lying low on this with good reason. They may be in the middle of an investigation which would be compromised if they spoke up.
If I had to guess, I would guess that someone hacked Hunter's email account or laptop and got legit emails along with the sex tape. Those got mixed in with fabricated emails -- and some of the emails are said to look fraudulent, lacking context. Others would be genuine, and other parties would say as much, "Yes, I sent that" or "Yes, I received that." That would give the collection of emails more credibility.
One thing we know is that Trump was warned that Giuliani was associating with a Russian agent. How would American intelligence know that? They probably wouldn't have learned it from spying on Giuliani. They would have been spying on the other guy and found out Giuliani was hanging out with him. Why would they tell Trump and not Giuliani? My guess is they thought maybe Giuliani was "colluding" with the Russians and Trump deserved to know. They wouldn't tell Giuliani since he might spill the beans and then the Russian agent would know he was being spied on.
Another thing which needs clearing up is what motivated the owner of the computer store to make copies of the hard drive. In one interview, that owner rambled on about Seth Rich and said just having the hard drive made him fear for his life. Giuliani said something similar on Fox.
Giuliani: Computer shop owner who found alleged Hunter Biden emails told FBI he was 'really afraid'
In an interview airing on Fox News' "Life, Liberty & Levin" Sunday evening, Giuliani -- now President Trump's personal attorney -- told host Mark Levin that the Delaware computer shop owner who allegedly took possession of Hunter Biden's laptop told an FBI agent he was "really afraid because the Bidens are very powerful around here."
"The gentleman who was repairing the hard drive -- and Hunter Biden abandoned it," recounted Giuliani, apparently referring to John Paul Mac Isaac. "He [Hunter Biden] may very well have forgotten where he left it for a while, because the gentlemen who got it said he [Hunter] was drunk for the entire time that he dealt with him, and he dealt with him twice."
I find that highly improbable. I know I wouldn't react like that. First, I wouldn't have the time to look at all the contents of a hard drive like that. Maybe the customer would come back to claim it. But if I did look at it and found dangerous stuff, I'd call the FBI immediately and hand it over to them. If I felt knowing something might endanger my life, I'd be begging the FBI to take it. I wouldn't be making copies of it to spread around, risking my name getting dragged into a public spectacle.
If Hunter Biden forgot where he left the laptop, why would I be worried for my life about it? Why would he abandon the laptop only to get so worried about it later that I should fear for my life? It might make a good movie, but I don't think it's very realistic.