Gnus will keep a Message-ID history file of all the mails it has sent. If it discovers that it has already sent a mail, it will ask the user whether to re-send the mail. (This is primarily useful when dealing with SOUP packets and the like where one is apt to send the same packet multiple times.) This variable says what the name of this history file is. It is `~/News/Sent-Message-IDs' by default. Set this variable to nil if you don't want Gnus to keep a history file.

This variable says how many Message-IDs to keep in the history file. It is 1000 by default.