Custom Domain Verification, DKIM Rotation, and Suppression for Transactional Email APIs

Short answer: Choose a transactional email API only after its custom domain verification, DKIM rotation, suppression export, event history, and rollback controls let a small team explain every accepted, deferred, bounced, or blocked message. A transactional email API is only simple while delivery state stays simple. The operational constraint is recovery, not the length of the send request. That is the choice. A low send price is useful, but it can't compensate for a sender identity nobody can rotate safely or a suppression list nobody can inspect. I've been paged for missed jobs and duplicate deliveries. Email creates the same class of incident: an application retries because it can't tell what happened, then either drops a message or sends it twice. Treat the provider as one part of a delivery system, not as a Send() function with a receipt. What should a startup verify in a simple transactional email deliverability API? Start with a short proof, using a subdomain that is separate from employee mail. Verify that the service can establish the custom domain through DNS records you control, show each record's status independently, and preserve the previous signing configuration while a new DKIM selector is being rolled out. A single green "verified" badge isn't enough evidence for a runbook. Then trace one synthetic message from the application's request ID to the provider's message ID and onward to the final event. The API should distinguish request acceptance from actual delivery. Those are different states, and collapsing them makes retry policy dangerous. Check how long event data remains queryable, whether webhook events can be replayed or recovered, and whether a human can export the same data during an incident. Suppression management deserves its own test. You need to know what creates a suppression, its scope, how it is queried, and what review is required before removal. An unsubscribe, a permanent delivery failure, and an operator block may all prevent a s