how to keep all threads?

OE 6: I have noticed that by default newsgroup threads do not stay with you
forever. I may flag (with the glasses icon) a message so that I can keep
track of it. I usually sort by this column so they all go to the top of my
list. I know that after not viewing a particular group for a while when I do
come back to view it I can see all my flagged threads disappear as the
screen refreshes with new downloaded headers.

I don't want that to happen. Can I configure OE to just keep *all* the
threads I've downloaded indefinitely. without having to save each one
manually? I often want to refer back to something that may be months or
years old.