Answer by quanta for grep out unique senders from a huge list of emails

Try this:

awk '/^From: / { print $2 }' /home/user/mail/new | sort | uniq -c | sort -rn

It's not one file, each email is about 20K, and the total emails amount to 30G.

awk '/^From: / { print $2 }' /home/user/mail/* | sort | uniq -c | sort -rn