Email API: What is it, and how is it useful?

What is an email API?

API is short for Application Programming Interface, where a set of rules or protocols are used to connect different systems. It helps one system to plug into the functionalities of the other. In the case of email APIs, the APIs allow applications to send, receive, analyze, or manage emails using the API request. It helps applications plug into and use the email functionalities of an email service provider. Email APIs are typically provided by third-party services like ZeptoMail, Zoho Mail, or SendGrid.

How does an email API work?

Key terms 

1. API endpoint: This is the URL exposed by the email provider that you’ll make API requests to. It provides access to the email provider's functionality. Example: https://api.example.com/products/.

2. HTTP methods: REST API calls can be made using different methods depending on what action is to be performed through the API. Here are the most commonly used methods:

   • GET: Fetch data

   • POST: Submit new data

   • PUT: Update existing data

   • DELETE: Remove data

3. Request and response: The client will initial an API request to the server. In the email API case, the request will contain the information of the email to be sent. The reply from the server once the request is processed, is the response.

4. Authentication: APIs often require API keys, tokens, or OAuth to ensure secure access and track usage. An API request to send an email looks like this (REST API):

   POST /v3/mail/send HTTP/1.1
   Host: api.zeptomail.com
   Authorization: Bearer YOUR_API_KEY
   Content-Type: application/json
   {
     "personalizations": [
       {
         "to": [{ "email": "user@example.com" }],
         "subject": "Welcome to Our Service"
       }
     ],
     "from": { "email": "noreply@yourcompany.com" },
     "content": [
       {
         "type": "text/plain",
         "value": "Thanks for signing up. We're glad to have you!"
       }
     ]
   } 

How the email API works

Let's look at how an API request is processed to send an email.

1. API authentication: The developer configures the app/client to authenticate using an API key or OAuth token.

2. Request submission: The client sends an HTTP request (POST) to the API endpoint (ESP) with payload data: sender, recipient(s), subject, content (HTML or plain text), and optional metadata.

3. Processing and delivery: Once the request is received, the email API service sets up email queuing, formatting, and delivery via SMTP or internal messaging systems.

4. Event tracking: Once the action is complete, in this case email delivery, the server (ESP) returns a status code to the client to inform them of success or failure.

A real-world example of how an email API works

Let’s look at an example of an eCommerce business. Here's how email API powers the order confirmation email process.

1. User performs an action.

   The customer purchases an item on an eCommerce website. After they hit "Place order", the website's backend system processes the order information and relays it to trigger the email-sending process through a send email API.

2. Email is constructed.

   Using the information from the website, the email content with all of the correct information about the order is prepared. For example:

   • Recipient: user@example.com

   • Subject: Zylker–Your order #1234 is confirmed

   • Email body: An HTML body with the order details, shipping address, total cost, and other important information.

3. API request is made.

   The eCommerce website makes an HTTP POST request to the email API provider. In this case, let's say that the provider is Zoho Zeptomail. This is what the API will look like:

   POST https://api.zeptomail.com/v1.1/email
   Authorization: Zoho-enczapikey YOUR_ZEPTO_API_KEY
   Content-Type: application/json
   { 
     "bounce_address": "bounce@yourdomain.com", 
     "from": { 
       "address": "orders@yourstore.com", 
       "name": "Your Store" 
     }, 
     "to": [ 
       { 
         "email_address": { 
           "address": "sarah@example.com", 
           "name": "Sarah" 
         } 
       } 
     ], 
     "subject": "Your Order #45678 is Confirmed!", 
     "htmlbody": "<p>Hi Sarah,</p><p>Thanks for your order #45678!</p><p>Order Total: $119.97<br>Shipping to: 123 Elm Street, Springfield, IL<br>Expected Delivery: July 10, 2025</p><p><a href='https://yourstore.com/orders/45678'>Track your order</a></p>", 
     "track_opens": true, 
     "track_clicks": true 
   } 

4. Request is processed.

   The provider performs the validation, inputs the dynamic data into the predefined template, and queues the email for delivery. The email delivery will be handled by the SMTP infrastructure of the email provider.

   Once the request is processed, the server will return a response to the client conveying the status of the request. Here are some of the common response codes:

   HTTP Status Code	Meaning
   200 OK	The request was successful.
   201 Created	The resource was successfully created.
   202 Accepted	The request has been accepted for processing (e.g., the email is queued).
   204 No Content	The request was successful, but there is no content to return (often used for successful deletes).
   400 Bad Request	There was a problem with the request (e.g., missing or invalid parameters).
   401 Unauthorized	Authentication failed; API key or token is missing or incorrect.
   403 Forbidden	The server understood the request, but access is denied.
   404 Not Found	The requested resource could not be found.
   405 Method Not Allowed	The HTTP method used is not allowed for this endpoint.
   422 Unprocessable Entity	The request was well-formed but contains semantic errors (e.g., invalid email format).
   429 Too Many Requests	Rate limit exceeded; too many requests sent in a given time frame.
   500 Internal Server Error	Generic server error; something went wrong on the server side.
   503 Service Unavailable	The service is temporarily unavailable (e.g., overloaded or under maintenance).
   504 Gateway Timeout	The server did not receive a timely response from an upstream server or service.

5. Email is delivered.

   The recipient will then receive the email in their inbox. If there is an issue with the email address or other reasons that the email cannot be delivered, then it is bounced.

6. Email is tracked.

   Because our API request includes open and click tracking, the email provider inserts tracking pixels and link wrappers into the email body. These elements will allow them to track opens, clicks, and delivery status to relay back to you. You can use the tracking APIs provided by the email service provider (ESP) to access them from your application if needed.

Email API vs. SMTP

Both email API and SMTP are reliable email delivery methods. Depending on how deep you want your integration with the ESP to go, you can pick email API or SMTP.

Configuring an SMTP connection is as simple as coping and pasting some credentials. SMTP works by opening a direct connection to the SMTP server and using the credentials provided to authenticate the connection. This works best if you are simply looking to send emails using the integration and don’t want to involve additional coding to the mix.

However, if you're looking for a deeper integration with the ESP and would like to access the full range of the functionalities available, it’s best to go the email API route. While it involves a certain amount of code changes, it will help you unlock advanced functionalities that go beyond just sending emails, like recipient activity tracking, templates, and more.

While both SMTP and email API are reliable, your choice will depend on what you're looking to achieve using the integration.

Types of email API

By content

Email APIs can be categorized into four main groups based on the type of email that is sent.

• Transactional email API: These email APIs handle automated, user-triggered notification emails like OTPs, password resets, and welcome emails. Zoho ZeptoMail provides transactional email APIs.

• Business email API: These are exposed by business email providers like Zoho Mail. While the client is used to send emails directly, these APIs are more often used to create integrations with the business inbox.

• Marketing email API: These email APIs are used to send bulk, high-volume marketing or promotional emails.

• Contextual email API: These APIs are designed to provide full email capabilities, essentially acting as an email engine to build your application on. Mail360 by Zoho is one such platform that can power your application's email workflow.

By purpose

Email APIs can also be categorized based on the type of function they perform or their purpose in the email life cycle.

• Send API: All email APIs that enable email sending fall under this category.

• Receiving API: These email APIs handle the receiving of the emails and further categorization or parsing.

• Tracking API: These email APIs are used to retrieve tracking information like recipient activity and other statistical data on the sent emails.

The benefits of email API

• Ready to use: Building an email infrastructure takes huge amounts of time and resources. With email APIs, you can simply plug into the functionalities of an email service provider without having to worry about the building time, cost, or maintenance.

• Customization: Email APIs allow for a deep integration with the email service provider. This will allow you to customize and personalize your emails to the extent required.

• Automation: Automating your email flow using email APIs will help reduce the strain on resources and minimize the margin of manual error.

• Deliverability: Email APIs come with built-in features that boost email deliverability, which will help deliver your emails better.

• Scalability: Email APIs are built to handle high volumes of email sending. Whether it's a small business or enterprise, email APIs are future-proof for scaling businesses.

• Security: Email APIs come with security features right out of the box. Using them to send emails helps secure your email sending.

How do you choose the right email API service?

Email APIs are provided by email service providers so you can plug into their functionalities. While there are many email service providers in the market, and the requirement of each business varies, there are some boxes that any email API service you choose should tick.

Deliverability 

The ESP should have the right verification and sending practices in place to ensure that the emails are delivered to the inbox instead of landing in spam or bouncing.

Reliability 

Your emails are important to your business, especially transactional emails. The email API service you use should have reliable uptime and consistent performance so there are no interruptions.

Security 

Emails will carry your business information and your customer's confidential details. The email API service you pick should have multiple layers of security in place to protect the emails, like encryption, data security, authentication, and more.

Compliance

Depending on your business's location and industry, you’ll have to comply with some regulations. Make sure that the ESP you pick abides by these regulations and helps you achieve compliance.

Ease of integration

It's no use finding a great email service if you have to spend a lot of time and effort figuring out how to connect with them. Pick an email API provider that supports the language of your choice and has a wide range of libraries to choose from. This will make the set-up easy.

Scalability

The volume of some emails, like transactional emails and marketing emails, will increase as your business grows. Ensure that the ESP you pick will be able to scale with your growth. This way, you won’t be forced to look for alternatives when the ESP can no longer handle your growing volume.

Features and customization

Every business has different requirements, so depending on your needs, make sure that the service you pick has all of the right features and has room for customizations in the future.

Support

Having a knowledgeable and easily accessible customer support team will ensure that your email sending runs smoothly. When it comes to delivering important emails, delays can be critical. Picking an ESP with good customer support can help avoid such delays. You should also ensure that the email APIs provided are supported by in-depth documentation on how to employ them. This will help save time and effort in setting the system up.

Pricing

Finally, what good is a great email service if it’s burning a hole through your pocket? Pick an email API provider that provides its services at an affordable price and doesn’t lock you into expensive plans and add-ons. Ensure that the pricing is flexible and at scale with your usage.   Zoho ZeptoMail is an email API provider who checks all of these boxes and more. It was purpose-built with a singular focus on automated email sending. While ZeptoMail also provides email sending through an SMTP connection, the robust set of email APIs provided allows for a deeper and more seamless integration with your website or application.