Borrowed idea from https://github.com/adipasquale/mbox-sender-frequency
Sample run:
➜ ./sender_frequency -threshold 500 -file ~/Downloads/Takeout/Mail/All\ mail\ Including\ Spam\ and\ Trash.mbox
"Amazon.in" <store-news@amazon.in>: 703
Groupon <noreply@r.groupon.com>: 741
"Flipkart" <mail@flipkartletters.com>: 729
YouTube <noreply@youtube.com>: 598
GitHub <noreply@github.com>: 1168