From Fan History Wiki
A mailing list is just that, a list of email addresses that are connected in order to share information and material between multiple recipients. Before LiveJournal, mailing lists were an incrediably popular means of communicating with other fans; fanfiction, fanart, essays, etc were shared across fandoms.
Mailing lists played an important role in fandom as early as 1992. They continue to play an important role in fandom.