How MIME and SMTP Work Together to Send Emails 📧

Have you ever wondered how emails are sent and received over the internet? How can you attach images, videos, or documents to your messages? How do email servers communicate with each other? In this article, we will explore the answers to these questions by learning about two protocols: MIME and SMTP.

What is MIME? 🤔

MIME stands for Multipurpose Internet Mail Extensions. It is a standard that defines how to format and encode different types of data for email transmission. MIME allows you to send not only plain text messages, but also rich text, HTML, images, audio, video, and other kinds of files as attachments. MIME also supports multiple languages and character sets, so you can write emails in any language you want.

MIME works by adding extra headers to the email message that describe the content type, encoding, and boundary of each part of the message. For example, a simple MIME message with a plain text part and an image attachment might look something like this:

From: [e-mail]
To: [e-mail]
Subject: Hello
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="abc123"

--abc123
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit

Hi Bob,

This is a test message with an image attachment.

Alice

--abc123
Content-Type: image/jpeg; name="cat.jpg"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="cat.jpg"

/9j/4AAQSkZJRgABAQEASABIAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRof
Hh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/2wBDAQkJCQwLDBgNDRgyIRwh
MjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjL/wAARCAA
...
--abc123--

The --abc123 is the boundary that separates the different parts of the message. The first part has a content type of text/plain and a content transfer encoding of 7bit, which means it is plain text encoded in ASCII. The second part has a content type of image/jpeg and a content transfer encoding of base64, which means it is an image encoded in base64. The content disposition header indicates that this part is an attachment with a filename of cat.jpg.

What is SMTP? 🤔

SMTP stands for Simple Mail Transfer Protocol. It is a protocol that defines how email servers communicate with each other to send and receive emails. SMTP uses a client-server model, where the client is the sender's email server and the server is the recipient's email server. SMTP uses commands and responses to exchange information and data between the client and the server.

SMTP works by following three stages:

1. Connection stage: The client establishes a TCP connection to the server on port 25 (or 465 for SSL/TLS encryption). The server responds with a greeting message that contains its domain name and capabilities. The client then identifies itself with the HELO or EHLO command, followed by its domain name. The server acknowledges the client's identity with a response code of 250.

2. Mail transaction stage: The client initiates a mail transaction with the MAIL FROM command, followed by the sender's email address. The server verifies the sender's address and responds with a code of 250 if it is valid. The client then specifies one or more recipients with the RCPT TO command, followed by the recipient's email address. The server verifies each recipient's address and responds with a code of 250 if it is valid. The client then indicates that it is ready to send the message data with the DATA command. The server responds with a code of 354 if it is ready to receive the data.

3. Data transfer stage: The client sends the message data, which consists of headers and body, followed by a line containing only a period (.). The server receives the data and stores it in its mail queue for delivery. The server then responds with a code of 250 if it has accepted the message successfully.

How MIME and SMTP Work Together 🤝

When you send an email with attachments or rich content, your email client (such as Outlook or Gmail) uses MIME to format and encode your message data according to the MIME standard. Then, your email client uses SMTP to connect to your email server (such as example.com) and send your message data to it. Your email server then uses SMTP to connect to the recipient's email server (such as example.net) and deliver your message data to it. The recipient's email server then stores your message in its mail queue until the recipient's email client (such as Outlook or Gmail) retrieves it. The recipient's email client then uses MIME to decode and display your message data according to the MIME standard.

Conclusion 🎉

In this article, we learned about two protocols that enable email communication over the internet: MIME and SMTP. We learned how MIME formats and encodes different types of data for email transmission, and how SMTP transfers messages between email servers using commands and responses. We also learned how MIME and SMTP work together to send and receive emails with attachments or rich content.

We hope you enjoyed this article and learned something new! If you have any questions or feedback, please leave them in the comments below 👇