Here are the mailing list addresses for sam, rc, and es. 9term is generally discussed on the sam-fans list.
Mailing list archives are all on ftp.sys.utoronto.ca.
The mailing list for Plan 9 is email@example.com which is the list itself, and firstname.lastname@example.org . This latter address is an automated mail server that will reply with a message telling you how to subscribe. The server also manages the archives. According to Daniel Ehrlich, the manager, “You can get a list of available files by sending e-mail to Majordomo@cse.psu.edu with index plan9-fans as the message body. Files can be retrieved with getplan9-fans filename. If you want a complete list of available commands send help to email@example.com .”