Hacking stdin? (Why doesn't this filter do what I expect)

It should be self explanatory how this filter should work. Right now I get
no body in the message returned. I don't understand why.

What is stdin within this filter?

I want to do this without relying on temporary files.

                :0f
                | $FORMAIL -rtz \
                        -I "Subject: Re: $SUBJECT [rejected]" \
                        -A "X-Loop: robert(_at_)elastica(_dot_)com" \
                        -I 'Content-Type: multipart/mixed; boundary="--
next item --"' ;\

                echo "This is the preamble of an RFC-1521 encoded, mixed
message." ;\
                echo "-- next item --" ;\
                echo "Content-Type: text/plain" ;\
                echo "" ;\
                $CAT $MESSAGE ;\
                echo "-- next item --" ;\
                echo "Content-Type: message/rfc822" ;\
                echo "" ;\
                $CAT - | $SED "s/\$SPAMCODE/$SPAM_CODE/g" ;\
                echo "-- next item --"