Sandbox mode
A starter mode that limits sending to verified addresses.
Some providers (most notably AWS SES) start every account in sandbox mode: you can only send to verified addresses until you request and receive production access. Plan for the approval lead time when launching.
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 sandbox mode, 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.