Deleting Expired Mail

[Eric Swenson <ejs@borland.com>]

I'm still confused about how the expiring and deleting works for mail
groups.  I'm running Gnus 5.2.22.  What I'd like to have happen is to
either manually or via some automatic rules, be able to mark messages
as expirable and then with no further action on my part, after a
certain period of time, have these messages actually expired and
deleted from my mail file.  I know that messages I mark with "E" can
be manually deleted from my mail file with the 'B ESC C-e" command but
I don't want to have to remember to type this in all my (many) mail
groups.  

I assume what I want is possible and every once in a while in some
groups, when I exit the group, instead of seeing "Expiring articles" I
see a message like "Deleting article NNN".  

Would someone please explain this process to me.  I have looked at the
Gnus 5.1 info pages (but don't have TeX so cannot build info pages for
the current version) and still do not understand this expiration
thing.

Thanks.  -- Eric