Warming Up

Warming up is the process of gradually increasing your email volume on a new domain to build sender reputation with mailbox providers like Gmail, Outlook, and Yahoo. You can connect AgentMail inboxes to Instantly and Smartlead for programmatic warm up using SMTP credentials.

Why warming matters

Mailbox providers track sender reputation per domain. A brand new domain has zero reputation. If you send thousands of emails on day one, providers will flag your domain as suspicious and route your emails to spam.

By starting slow and increasing volume gradually, you signal that you are a legitimate sender with engaged recipients.

Warmup schedule

Tips for warming with agents

Spread sends across multiple inboxes. Instead of sending 100 emails from 1 inbox, send 10 emails each from 10 inboxes on the same domain. AgentMail makes it easy to create many inboxes, and this approach distributes volume naturally while warming the domain faster.

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import { AgentMailClient } from "agentmail";
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const client = new AgentMailClient({ apiKey: "am_..." });
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// Create multiple inboxes on your domain
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const inboxes = [];
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for (let i = 0; i < 10; i++) {
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  const inbox = await client.inboxes.create({
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    username: `agent-${i}`,
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    domain: "yourcompany.com",
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    clientId: `warmup-agent-${i}`,
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  });
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  inboxes.push(inbox);
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}
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// Distribute sends across inboxes
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for (const [index, recipient] of recipients.entries()) {
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  const inbox = inboxes[index % inboxes.length];
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  await client.inboxes.messages.send(inbox.inboxId, {
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    to: recipient,
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    subject: "Hello",
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    text: "Plain text version",
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    html: "<p>HTML version</p>",
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  });
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}

Start with replies, not cold outreach. If possible, have your agent respond to inbound emails first. Replies build reputation much faster than cold sends because they demonstrate two-way engagement.

Use multiple domains and subdomains for high volume. Register multiple domains or use subdomains (e.g., mail.yourcompany.com, outreach.yourcompany.com) and warm each one separately. This protects your primary domain’s reputation if one sending domain gets flagged.

Only send to verified addresses. During warmup, never send to purchased lists or unverified addresses. High bounce rates during the warmup period will severely damage your new domain’s reputation.

Key metrics to monitor

You can track bounces and complaints by using Query Metrics in AgentMail’s API or by viewing the Metrics Page on the console.

For more deliverability best practices, see the Email Deliverability guide.