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 nobod...