Why are my emails bouncing?

Fresh 🌱

A bounced email means the recipient's mail server rejected your message. Understanding the bounce type helps you take the right action.

Bounce types

Permanent bounces

The address is permanently unreachable. Common causes:

• The email address does not exist
• The domain does not exist
• The recipient's mailbox has been deleted

Action: Remove permanently bounced addresses from your sending lists immediately. Continuing to send to them will damage your sender reputation.

Transient bounces

Temporary delivery failure. Common causes:

• Recipient's mailbox is full
• Recipient's mail server is temporarily unavailable
• Message is too large
• Greylisting (server temporarily rejects first-time senders)

Action: AgentMail automatically retries transient bounces. If delivery fails after multiple attempts, the bounce is treated as permanent.

Monitoring bounces with webhooks

Subscribe to the message.bounced webhook event to track bounces in real time:

import { serialization } from "agentmail";

async function handleWebhook(payload: Record<string, unknown>) {
  if (payload.event_type === "message.bounced") {
    const event = await serialization.events.MessageBouncedEvent.parse(payload);

    console.log(`Bounce type: ${event.bounce.type}`);       // "Permanent" or "Transient"
    console.log(`Sub type: ${event.bounce.subType}`);        // e.g., "General"
    console.log(`Message: ${event.bounce.messageId}`);

    for (const recipient of event.bounce.recipients) {
      console.log(`Bounced address: ${recipient.address}`);

      if (event.bounce.type === "Permanent") {
        // Remove from your sending lists
        await removeFromList(recipient.address);
      }
    }
  }
}

Register a webhook to receive bounce events:

import { AgentMailClient } from "agentmail";

const client = new AgentMailClient({ apiKey: "am_..." });

await client.webhooks.create({
  url: "https://your-domain.ngrok-free.app/webhooks",
  events: ["message.bounced"],
});

Automatic suppression

AgentMail automatically prevents you from sending to addresses that have previously bounced, been rejected, or filed a spam complaint. This protects your sender reputation. Keep your account bounce rate under 4%, otherwise your account may be placed under review.

Keeping bounce rates low

Practice	Why it matters
Validate addresses before sending	Catches typos and nonexistent addresses before they bounce
Remove permanent bounces immediately	Repeated sends to invalid addresses damage reputation fast
Avoid purchased or scraped lists	These lists have high rates of invalid addresses
Warm up new domains gradually	Sudden high volume from a new domain triggers suspicion
Monitor bounce rate	Keep it under 2% for healthy reputation; under 4% to avoid account review

For more on maintaining healthy sending metrics, see the Email Deliverability best practices.