11 Feb 2015 11 Feb '15
http://www.mail-archive.com/faq.html specifies "X-No-Archive: yes"
It's been a long time, but I vaguely remember lowercase 'yes' being canonical which is why we specify it that way in the FAQ. In practice mail-archive.com treats the header as case insensitive.
Here's some data on common practices, taken from a recent week of mail-archive.com's inbound mail.
$ zgrep -i "^x-no-archive:" *.mbox.gz | cut -f3 -d":" | sort | uniq -c
72 no 183 No
6407 yes 108 Yes 47 YES