DKIM sign only unsigned

[Damian]

> I don't. Advise?
>
> client IP does not work since the same IP sometimes send signed, 
> sometimes
> unsigned mail.
>
> That's why I ask about only signing mail that is not signed, or, skipping
> mail that is already signed (with valid signature).
Along the lines of:

> $enable_dkim_signing = 1;
> $enable_dkim_verification      = 1;
> @author_to_policy_bank_maps    = 'HAS_VALID_DKIM';
> $policy_bank{'HAS_VALID_DKIM'} = { enable_dkim_signing => 0 };