[exim] Adding authentication-results header field only local…

Hi,

I'd like to add the Authentication-Results: header field only to
messages that are being delivered locally; it doesn't make sense for me
to attach it to outgoing messages.

I started out by putting in the acl_check_data ACL:

  warn
    add_header     = :at_start:${authresults {$primary_hostname}}

and that works fine, except it includes it in outgoing messages too.

So instead I have tried putting into my mail_spool transport:

headers_add = ${authresults {$primary_hostname}}

but I'm not seeing the header field in locally-delivered messages (yes,
I am using mail_spool local delivery). Is this the wrong approach?

Once I get this working I'd like to do the same with the spamassassin
header fields.

-Jim