[exim] Run a command whenever a specific authenticated user …

Hi,

I share my exim mail server with other users.
Whenever I send a mail (I can be identified by my authentication) I
would like to put the "To" header in my addressbook using an external
command.

To test it, I tried this:
acl_check_data:
  warn  authenticated = nathan@???
          message = ${run{echo $h_To >> /tmp/local}}

nathan@??? is my smtp login name.
But /tmp/local does not appear when I send a mail.

Other ideas of how to do this?
Thanks!
Nathan
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