Including Recipients in DKIM Signatures
Murray S. Kucherawy · 2016
The DomainKeys Identified Mail (DKIM) protocol applies a domain-level cryptographic signature to an e-mail message. DKIM only guarantees authenticity of the message content and does not consider the message envelope. This allows for replay attacks by recycling a signed message with an arbitrary new set of recipients. This document presents a protocol extension that can include the original set of envelope recipients in the signed content, so that an altered set of recipients renders the signature invalid.