Well, those investigating Weiner are probably not limiting their search to only emails with obvious subject headers. Huma had testified she did not do inbox maintenance, she left everything in the inbox or on the desktop. Investigators would be seeing an abundance in plain sight.
But. They were probably recovering deleted messages too. If this is where the emails in question were discovered. Well, that changes everything.
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.