I know that an emails' sender can be verified e.g. through DKIM.
If I receive an email chain containing replies, forwards, etc. is there any way to verify the sender of other (earlier) emails in the chain?
If you get a mail with was 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. For other kinds of forwarding (i.e. quoted, prefixed...) this is not possible. It is also not possible for replies since none of these contains both the necessary
DKIM-Signature header and the unchanged mail headers and body which are covered by this signature.