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 teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to dkim, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.