Forwarding mail from user+foo to foo

Howdy!

I'm trying to write a procmail recipe that, when it receives mail
to me (rsr) of the form rsr+<foo>, will

A) If <foo> is one of a small number of keywords, do something
specific (for example, rsr+pgp will send my public key).
B) Otherwise, forward the mail to <foo>

Now the problem is:
A) I know how to make delivering recipes non-delivering, but I
don't know how to do it the other way around -- mark a
non-delivering recipe as delivering (so processing stops); 
B) When I tried this:
--
MTO = `/usr/local/bin/formail -xTo | sed 's/rsr\+//' | sed 
's/\(_at_)(_dot_)*//'`

:0
* ^To:.*rsr+
| /usr/local/bin/formail -c -z -i "To: $MTO" | $SENDMAIL -oi -t 
--

I kept getting weird mail bounces (mostly mail loops).

What am I doing wrong?

-roy
           Roy S. Rapoport / rsr(_at_)macromedia(_dot_)com / 415-252-2215
                         Save the Mac, kill Apple