A mailing list is an email list built around a specific theme. There are literally hundreds of mailing lists on as many topics. Some lists are very slow -- 5-10 messages a day or less. Some are very active -- up to a hundred messages a day. Choose wisely -- when I first discovered mailing lists, I was getting as many as 300 email messages a day.

Shamash hosts many Jewish lists and has instructions on how to subscribe.
BRIDGES: the Jewish Feminist journal (subscribe)
RENEW-J: The general Jewish Renewal mailing list (subscribe)
GAYJEWS: Facilitate communication among Lesbian and Gay Jews (subscribe)
KOL-ISHA: Halachic questions and issues concerning women's roles in Judaism (subscribe)
MAIL-HAVURAH: National Havurah Committee Mailing List (subscribe)
NICEJG: Discussion list for self-identified lesbian and bisexual Jewish women (subscribe)
RECON-DT: Reconstructionist Divrei Torah (subscribe)
TREK-COCHAVIM: Star Trek from a Jewish cultural religious perspective (subscribe)
WELLSPRINGS: Wellsprings: Reconstructionist (subscribe)
HILLEL- WOMEN -- a women's issues discussion list (subscribe)
WOMENRAB: Women Rabbis and Women Rabbinical Students (subscribe)
Tor-Ch: Torat Chayim (Living Torah) explores the relationship between our world and the tradition in the spirit and context of Conservative Judaism (subscribe)
Rabin Mishnah Study Group, dedicated to the daily study of Mishnah by lay people (in memory of Yitzchak Rabin z"l) in the spiritual climate of Conservative Judaism (subscribe)