Thanks, Yoma!
After a decade of using Gmail,, getting tons of "promotions" I'm rarely interested in, Ihave over 25000 unread emails! I've done a program to download header info into a CSV (similar to what you've done), and needed to know how to send the unwanted ones to Trash once I've sifted through them. Your tips on the specifics of doing it the Gmail way will make that easy.
I ran into annoyances with MIME-encoding and embedded line terminators in From and Subject fields that I overcame. I'd be happy to send you a copy of my code if you'd like to use my clean-ups.
Thanks again,
Ran