DKIM
A cryptographic signature attached to outbound mail.
DomainKeys Identified Mail signs each message with a private key. Mailbox providers fetch the public key from DNS and verify the signature. DKIM proves the message was not tampered with and is critical for DMARC alignment.
reading this as developers picking email infrastructure
The question here is which one a backend engineer would rather operate. That means SDK depth in the language already in use, whether a retry can safely be replayed, how much the event log tells you when a customer says the email never arrived, and whether the pricing curve stays sane as volume grows.
Applied to dkim, that means weighing developer experience, SDK breadth, idempotency keys, and operating track record ahead of the rest, against password resets, receipts, and magic links shipped from application code.