November 2023 Release Notes

Throttling notices in every inbox

Custmers will start seeing informational messages in the inbox view{{ email address }} when there has been throttling imposed on the inbox or sender to the inbox.

Paying customers will very rarely experience throttling. In almost all cases, throttling happens because they were sending to a public inbox, not a custom domain or private address.

We only throttle incoming messages to protect the stability of our service for all of our customers. These protections have been in place for years, but were not transparent to customers.

If you are on a paid plan and you are seeing throttling messages, reach out to [email protected], we can help you configure a custom domain or private addresses. These both can be done in seconds with no need for DNS changes.

If you are on our free tier and seeing these messages, this is a nudge for you to sign up for a paid plan. We would love to have you as a customer.

Email Delivery Testing: Node.js, Cypress, and Mailsac

Email delivery testing is an essential part of ensuring your application sends emails correctly and efficiently. With Mailsac, you can take advantage of its powerful REST API to simplify your email testing process. In this blog post, we’ll show you how to use Cypress, a popular end-to-end testing framework, in combination with Mailsac for seamless email delivery testing. We’ll cover setting up the Cypress environment, running tests, and provide plenty of code samples to get you started.

Setting up the Cypress Environment

First, you’ll need to have Node.js installed on your computer. Once that’s done, follow these steps to set up Cypress:

1. Create a new directory for your project and navigate to it in the terminal. b. Run npm init to create a package.json file. c. Install Cypress by running npm install cypress. d. Add a script to your package.json file to run Cypress:

"scripts": {
  "cypress:open": "cypress open"

2. Configuring Cypress and Mailsac API

Next, create a cypress.json file in your project’s root directory. This file will store your Mailsac API key and other configuration options:

  "baseUrl": "",
  "env": {
    "mailsac_api_key": "your_mailsac_api_key"

Replace your_mailsac_api_key with your actual Mailsac API key.

3. Writing Your First Cypress Test

Now, let’s create a test file in the cypress/integration folder. Name it email_delivery_spec.js. In this file, we’ll write a test that sends an email to a random Mailsac address and then checks whether the email was received.

// cypress/integration/email_delivery_spec.js

describe("Email Delivery Test", () => {
  it("sends an email and verifies its receipt", async () => {
      const randomEmail = `test-${Math.random().toString(36).substring(2)}`;
      const testSubject = `Cypress Email Delivery Test ${Math.random().toString(36).substring(2)}`;
      const testBody = "This is a test email sent using Cypress and Mailsac.";

      // Send an email using your application's email sending method
      // ...
      // TOOD: integrate your app here!

      // Function to poll Mailsac API for received messages. It will be called
      // recursively.
      const checkEmail = async () => {
        let response = await cy.request({
          method: "GET",
          url: `${randomEmail}/messages`,
          headers: {
            // Get a mailsac api key at:
            "Mailsac-Key": Cypress.env("mailsac_api_key")
        const messages = response.body;
        const message = messages.find(msg => msg.subject === testSubject);

        if (!message) {
          return cy.wait(1000).then(() => {

        expect(message.from[0].address).to.equal("[email protected]");

        // Check email content for testBody text
        const textResponse = await cy.request(`${randomEmail}/${msg._id}`);

        // Clean up by deleting the received messages. This could also be done in an afterEach block.
        await Promise.all( =>
            method: "DELETE",
            url: `${randomEmail}/messages/${msg._id}`,
            headers: {
              "Mailsac-Key": Cypress.env("mailsac_api_key")

      // Start polling for received messages
      await checkEmail();

Replace [email protected] with your application’s sender email address.

This will open the Cypress Test Runner, and you’ll see your email_delivery_spec.js test listed. Click on the test to run it.