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 teams building autonomous agents
An agent stack treats email as a two-way primitive. Inbound has to arrive as structured JSON a tool call can consume, sends have to be safe to retry when a model loop repeats itself, and every action needs an audit trail. A provider without inbound parsing only solves half the problem.
Applied to sandbox mode, that means weighing inbound parsing, idempotency keys, webhook coverage, and operating track record ahead of the rest, against inbound parsing into tool calls and model-generated sends that must not duplicate.