If you read the document posted by FredZarguna around page34-35 it talks about “plain view”. Some courts have held that each individual file is a separate entity.
Where I disagree with FredZarguna is this was a sexting case which would normally involve texts or emails from Anthony to the 15-year old female and there would be a sender and receiver clearly marked in the header. While the agent could, under plain sight open the first Huma/Hillary email, he couldn’t open all of them without a search warrant.
It is highly unlikely that Anthony Weiner would use his wife’s email address to either send or receive messages from the 15-year old. This would limit the scope of the search and although they could use the first Huma email as probable cause for obtaining a search warrant, they don’t have license to open all.
This is according to the US Attorneys manual for Computer Crime FredZaguna posted.
But more importantly, depending on both the email client and its configuration, and the nature of their account access, it's conceivable that sender, recipient, date, and the first few header lines of every email on the laptop are all visible in a single pane, used to drill down to the full detail.
In such a case, literally thousands of emails would all have some of their most damaging information in plain view, intermixed between Huma's content and Weiner's. And there is no ambiguity in the case law about that. It's all admissible, even without a warrant.
“....there would be a sender and receiver clearly marked in the header...”
There would be a sender/receiver but quite possibly that same sender/receiver could be operating under a plethora of monikers which would necessitate looking through more than the obvious.
For the evidence to be considered tainted, wouldn’t it depend on who stumbled upon it? The only roll they are playing is whistleblower, and possibly witness. Say if it’s Family Services or some other state agency who alerts the FBI.