gnus-mailing-list-groups

gnus-mailing-list-groups is a variable defined in `gnus-msg.el'.
Its value is

nil

Documentation:
*If non-nil a regexp matching groups that are really mailing lists.
This is useful when you're reading a mailing list that has been
gatewayed to a newsgroup, and you want to followup to an article in
the group.

You can customize this variable.