**Cargo Mailboxes** is a feature of 📝Cargo that gives a workspace its own sending domains and inboxes, so plays, tools, and agents can deliver cold email through the native sendEmail action with pacing, suppression, and credit tracking handled by the platform. It exists because cold outreach from freshly registered domains needed a separate sequencer and rented inboxes, with replies and unsubscribes landing outside the system that decided to send. Built for 📝Go-To-Market (GTM) teams running outbound from Cargo, it is positioned for new-domain cold outreach rather than as a replacement for connected Gmail or Outlook accounts or transactional providers.

Key Capabilities

• Owned domains and inboxes — defineDomain adopts or registers a sending domain and defineMailbox mints a Google, shared, or private SMTP inbox on it; domain, username, and type are create-only.
• Warm-up ramp and pacing — the daily ceiling climbs from 5 to 40 sends over 45 days after warm-up starts, and a spread policy paces one send every 24 hours divided by the ceiling.
• Native sending — sendEmail takes a mailbox, recipient, subject, and HTML body, threads replies through inReplyTo and references, and bills 0.1 credits per delivered email.
• Workspace-wide suppression — unsubscribes, bounces, and manual entries opt a recipient out of the whole sender, and a suppressed address is refused before any message row is written.
• Thread events and triggers — sent, opened, clicked, replied, and unsubscribed land on the thread, and waitEmailEvent or an email trigger wakes a workflow or agent on reply.

Getting Started

1. Buy or adopt a sending domain, then declare it with defineDomain(domain, {adopt: true}).
1. Quote the monthly mailbox charge with cargo-ai mailboxManagement pricing get before deploying.
1. Declare the inbox with defineMailbox using a real person's name and local part; deploy.
1. Start warm-up: cargo-ai mailboxManagement mailbox start-warmup <uuid> on the same day.
1. Send a test to yourself via sendEmail and confirm threading, tracking, and the unsubscribe link.

Related

• 📝Cargo CDK
• 📝Cargo CLI
• 📝Smartlead

Contexts

• 🏷️#cargo
• 🏷️#feature-of-cargo