Rules: Do action on the field of referenced node I have an "Authors" Content-type, that users can add to their "Article" node while adding a new content as Co-Authors.

Note that it's a seperate content-type not users.

in "Authors" content-type I have an "Email field" which contains the email of the co-author.

What I want to do is using rules, send an email to the referenced node's email field

I juggled around in rules but couldn't really make it work, I'd really appreciate a step-by-step process of rules to achieve this, thanks.
If your rule is set to React on event: Before saving content or After saving new content of type Article, then do this:

Add condition: Entity has field

ENTITY: node:field-your_reference_field

FIELD: field_email

This makes the referenced node's email field available in to Actions. So then you can

Add action: Send email

To: node:field-your_reference_field:field_email

This should get you where you need to be.