One-click unsubscribe
A standards-based unsubscribe action in the inbox UI.
One-click unsubscribe uses List-Unsubscribe and List-Unsubscribe-Post headers so supported mailbox providers can show an unsubscribe control directly in the inbox. Gmail and Yahoo require it for many bulk senders.
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 one-click unsubscribe, 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.