Header-People
The ARPANET list where the email header format was argued into existence — usually in very long messages.
In the mid-1970s every ARPANET host had its own idea of how to write From:, Date: and addresses. Header-People was the mailing list where the people building mail programs tried to agree. The debates fed into RFC 733 (1977) and later RFC 822 (1982), the format email still follows.
The list is at least as famous for its tone. Questions like how to write a date, or whether an address may contain spaces, turned out to sustain weeks of traffic, and Header-People is often cited as one of the birthplaces of the flame war as a genre. That the standard ended up so tolerant of odd headers is partly a peace treaty.