dmarc - How to set Exim envelope domain to From domain

I've set up DKIM on Exim with the domain set like:

DKIM_DOMAIN = ${sender_address_domain}

However, the domain is always set to the same domain (my primary domain), which causes DMARC validation to fail, because of alignment, when sending emails for other domains (I host several websites).

From reading the documentation, I think the sender_address_domain is the envelope address and not the From field. How can I change the envelope address so that it matches the From field of a given email (I assume this will also allow SPF alignment to be correct)?

Also, for security, is it possible to have a whitelist of allowable domains, so Exim refuses to send emails that have another domain in the From field?

Answer

Add the rewrite rule:

*       "$header_from:" F

In debian this can be added by creating a file such as /etc/exim4/conf.d/rewrite/10_from_rewrite. This rule rewrites the sender field to match the From header, allowing DMARC alignment to work correctly.