Emails can be roughly separated in two parts: The email message itself and some additional addressing information which is used to guide the message from the sender to the recipient. The email message standard is defined in RFC 5322 also known as IMF or Internet Message Format.
Consider the following diagram to see more specifically how RFC 5321 and RFC 5322 relates to email delivery and the email message:
SPF and DKIM are both used to perform email authentication but they are securing different parts of the email: