[exim] retry rules help...

Hi,

I've set up a secondary MX using exim and @mx_any to route mail to my
primary MX for all our domains. I'm finding that my queue is filling up
with frozen messages already and I'd like to control more accurately
when they are bounced.

What I'd like to achieve is:

* If the message is queued and the primary MX is unavailable (either
connect refused or timed out) then the message should stay in the queue
for 7days.

...this bit seems easy enough, BUT

* if the primary MX IS available but says "550 User unknown", I'd like
to bounce the message immediately (so induce a permanent failure).

I've been reading the manual about retry rules for specific errors and
the closest I can find is the example...

"the.host.name rcpt_452 F,1h,10m"

..is there a similar retry condition for "550 User unknown"?

Can anyone tell me if what I'm attempting is possible and if I'm on the
right lines with this.

Thanks, Adam.