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 early-stage startups
At an early stage the deciding factors are what the free tier actually covers, what the first paid invoice looks like, and how much of a founding engineer's week the setup costs. Feature depth that only matters at a million sends a month is noise until it is not.
Applied to dkim, that means weighing free tier, best published rate per 1,000, entry price, and developer experience ahead of the rest, against a waitlist, a launch announcement, onboarding email, and the first transactional sends.