Apache project mailing lists can add unsubscribe footers to messages. E.g. From spark-user. https://mail-archives.apache.org/mod_mbox/spark-user/201511.mbox/%3C5637830F.3070702%40uib.no%3E
If no one objects I will file an INFRA ticket to add the footer to user@h.a.o. Unsubscribe requests are less frequent on the dev mailing lists so we can leave those alone.
