[Exim] How to use script to validate address

Hello,

I would like to configure Exim to become an "SMS gateway".
It should proceed mail addressed to
xxxxxxxxxx@???
where xxxxxxxxx is a GSM telephone number. I cannot
use any list, because there are lots of possible numbers,
but I can do a script that can check telephone number validity
and eventually send SMS.
So Exim should do the following things:
if the mail address is not local, pipe it to the script.
The script is able to send the mail as SMS if the number
is valid.
If the script returns non zero (invalid number),
go on proceeding the mail normal way - probably resulting
in "Unknown recipient" or something.
Is it possible to do it somehow?


Thank you,
Jan Martinek