[Exim] relaying mail inbound to a LAN

hello list

I have looked in the faq for an answer to this problem, but the faq
seems to answer relaying to other domains, not to sub-domains.

Here is the problem:

there is a cablemodem with one (real) IP address. port 25 is forwarded
across that so smtp traffic arrives at 192.168.0.5. NAT is running.
Exim is running on this IP address. Let's call this machine freebsd.

Connected, on occasion, to the same network is a laptop (also a freebsd
box but running sendmail). This has an IP address of 192.168.0.10. Let's
call this machine laptop.

So we have 2 machines, of domain (for example) sample.whatever:
DNS looks like this:

sample.whatever IN A real.world.external.ip
freebsd.sample.whatever IN A real.world.external.ip
laptop.sample.whatever IN A real.world.external.ip

real.world.external.ip is the same

MX looks like this:

sample.whatever MX 20 my.backup.mx
sample.whatever MX 10 sample.whatever

this is the same for freebsd and for laptop (same mx)

When mail for laptop.sample.whatever arrives at freebsd.sample.whatever
I want it to hand it off to laptop.sample.whatever if laptop is
connected, otherwise keep it in mailstore (for say 30 days) then bounce
if it isnt handed off after that time period.

The exim box (freebsd) is handling a couple of domains hosted on the
same box rather well. There must be instructions on how to do this, but
I can't see them. Can someone point me in the right direction?

thanks
--
John - jfm@??? - http://www.reiteration.net/~jfm
For PGP public key finger jfm@??? or see webpage