newbie question on procmail

[M.W.Chang]

On one possibly want to answer this question, but I will try. It's not
fun at all as far as I have learned.. :)

In the linux=sxs section for procmail, the example rcfile sends all
messages into mailbox file.

If I want to keep them in the spool (for pop3 access), what changes
should I made to that example rcfile?

In the man file, I noticed this:

--- begin quote ---
:0                              # sink all junk mail
* ^Subject:.*junk
/dev/null

:0                              # pass along all other mail
! -oi -f "$@"

Do  watch out when sending mail from within the
/etc/procmailrcs/some.rcfile, if you send mail to addresses which match
the first rule again, you could be creating an _endless_mail_ loop.
--- end quote ---

Should I place the 2nd rule in the example procmailrc after $DEAULT?
And  what exactly was the warning about?