The official Apache archives:
For browsing: http://mail-archives.apache.org/mod_mbox/#hadoop
Raw mbox files: http://hadoop.apache.org/mail/
Several third parties provide excellent interfaces for searching or browsing the Hadoop mailing list archives:
The Mail Archive (Search for "Hadoop")
Search-Hadoop.com - searches all Hadoop mailing lists, Wikis, web site, source code, javadoc, and JIRA
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