Reference
Sandbox workspaces send only to members
Until production access is approved, a workspace can send only to its members' account emails on every sending path, and campaigns are blocked.
Updated Sep 27, 2026
ChangedWorkspacesEmail APISMTP relayCampaigns
The sandbox rule now applies to the API, SMTP, Compose, retries, automations and campaigns. Sending to anyone else returns 403 with the code unverified_workspace_recipient from the API and 550 from SMTP, and campaign sends are blocked. Workspaces with production access aren’t affected.
To lift the restriction, verify a domain and request production access.