You'll get a confirmation and you'll need to send one of them back to confirm.
Instructions are in the messages.
This is more than enough for all normal messages. Imagine a mailing list of a thousand people and someone sends a 1Meg picture or WAV or something to the list. That's One GIG of traffic for that one message. This is in place because the server will consider anything that has such a long list of recipients or cc's to be spam. This is version 1.94.4 of Majordomo.