Include Spam Headers on outbound mail

Hi,

I have a requirement to included spam headers on outbound mail and to have it log correctly.
I would like to know is there a way to do this without amavis seeing all mail as local.

I have reviewed the parameter @local_domains_acl = ( ".$mydomain" );
I changed the parameter to @local_domains_acl = ( "." );

So my understanding is that this parameter will make amavis see all domains as local and will therefore add the spam headers to all mails.
But this also logs all outbound mail as local mail.

Is there a way for amavis to include spam headers on outbound mail without it been seen as local mail?
Is there another similar parameter for outbound?

Regards,
Paul