[exim] Auto whitelisting - transport filters ?

Hello,
I want to keep a record of all recipient emails sent on the remote
smtp, for white-listing purposes.

Shadow transport would be ideal but it cant be used on non local
transports - why ?

So I tried

remote_smtp:
driver = smtp
transport_filter = /usr/exim/whitelist_address $recipient_addresses

AND $recipients
Those variables don't work in a transport. - Why ?

The environment variables are not setup with any data for a transport
filter.

SO
remote_smtp:
driver = smtp
transport_filter = /usr/exim/whitelist_address

and I am just going to pattern match the TO line out of the message
in the script.

BUT this is seriously sub optimal, what I want to do is some how pass
the envelope-to(s) to my script, OH and ideally it would only be
called if the transport succeeds like shadow transports and unlike
transport filters.

PS I do not want to grep the log file on a cron job, I prefer
immediate response.

Any Other ideas ?

TIA
Stuart.