Verifying Senders of Earlier Emails in a Chain
If you receive an email chain containing replies, forwards, etc., it may be possible to verify the sender of other (earlier) emails in the chain, depending on the method of forwarding used.
If the email is forwarded as a message/rfc822 attachment with all headers instead of inline, it is possible to validate the DKIM signature and check the result against a DMARC policy. However, for other kinds of forwarding, such as quoted or prefixed forwarding, this verification method is not possible. Additionally, it is not possible to verify the sender of replies, as they do not contain both the necessary DKIM-Signature header and the unchanged mail headers and body required for this verification.