Free accounts get a developer sandbox
Accounts without a paid plan are now Free accounts, and the sandbox rules are enforced everywhere, including over SMTP: a Free account can send only to its own address and to recipients that have confirmed.
Paid and trial accounts are not affected.
What a Free account gets
- A developer sandbox on your own domain. Add your domain and finish the DNS steps (ownership, DKIM and SPF). When all three are live, a Developer Sandbox Motor Block appears on that domain automatically. Generate its SMTP password or API key from Motor Blocks.
- A small domain allowance. The Domains page tells you what it is for your plan.
- A Free panel in the dashboard with your recipients, your current limits and your usage. Limits are shown there, not listed here, because they can change.
Adding recipients
Add an address in the Free panel and we email it. Nothing is sent to it unless the recipient clicks Review and allow; opening the link alone confirms nothing. Every one of those emails also carries Stop emails from this account. A recipient who uses it can never be emailed from your account again, and that cannot be undone from the account.
What happens when a send is not allowed
The sandbox behaves differently over the API and over SMTP, as it always has:
| Where | Recipient not allowed | Monthly sandbox limit reached |
|---|---|---|
POST /v1/send | Delivered to your own account address instead. The response's sandboxRedirect names the address you asked for and the one used. | 429 with code: "sandbox_monthly_limit" |
| SMTP | Refused with 550 5.7.1, which says why. Nothing is queued. | Refused with 550 5.7.1 |
Blocks created before an account became Free are held to the same recipient rule and to hourly, daily and monthly caps, and they are refused, never redirected: 403 with code: "free_recipient_not_confirmed", or 429 with free_limit_hour, free_limit_day or free_limit_month and a Retry-After header; over SMTP, 550 5.7.1 or a temporary 451 4.7.1. Adding a domain beyond your allowance returns 403 free_domain_limit.
What you need to do. If you send from a Free account by script or agent, add and confirm your test recipients in the Free panel first, and handle the code field (and sandboxRedirect in the /v1/send response). To send to anyone, upgrade and convert the sandbox to production (POST /api/developer/sandbox/convert). See AI agent quickstart.