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Table of contents
What Is MIME Protocol?
Why Do We Need MIME?
How Does MIME Work?
Features of MIME 
Purpose and Functionality of MIME
MIME with SMTP and POP
MIME Header Fields
Content-Type Encoding
Content Id
Content description
Advantages of MIME Protocol
Frequently Asked Questions
What is a MIME type?
What are the principles of MIME?
What is the MIME format for email?
What is the port number for MIME?
What does the MIME boundary mean?
Last Updated: May 31, 2024

Multipurpose Internet Mail Extension (MIME) Protocol

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Ashwin Goyal
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MIME is used in conjunction with SMTP (Simple Mail Transfer Protocol) and POP (Post Office Protocol) to allow email messages to include non-text data such as images, audio, and video. 
An email protocol is a process of establishing a communication channel between two computers for sending and receiving email. A mail server and two computers are involved when an email is transferred. Email uses a variety of protocols. By establishing connections and allowing users access to emails, these protocols differ. Here we will be focusing on the MIME protocol.

MIME Protocol

What Is MIME Protocol?

Multipurpose Internet Mail Extensions (MIME) is the acronym for Multi-Purpose Internet Mail. It's an extension of the Internet email protocol that lets people send, receive, and exchange different data files, such as images, audio, and video, over the Internet. 

  • MIME allows users to send emails with special elements like non-ASCII characters and send more diverse content through the email system. For example, whenever we send an email with an attachment, MIME encoding is used to package the data and transmit different parts of the email in a specific way that can be interpreted by the receiver of the mail.

Why Do We Need MIME?

Now that we know about the MIME protocol, in this section we will look at the need of MIME protocol. Below are some of the major reasons of using MIME protocol

  • MIME is necessary to be used to send special Non-ASCII characters through emails.
  • MIME format is used for nearly all manual and automated Internet emails transmitted by SMTP. 
  • MIME standards also define content types that are essential in other communication protocols like HTTP

Nathan Borenstein asked the IETF of Bellcore in 1991 to extend SMTP to recognize and handle different kinds of data, besides ASCII text, so Internet clients and servers could recognize and manage such data. Therefore, new file types are now supported through "mail" as an Internet Protocol file type.

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How Does MIME Work?

A MIME-formatted email can be sent via standard protocols such as SMTP, POP, or IMAP. These emails may contain multiple parts. It is possible to specify the header in non-ASCII character sets supported by MIME. The MIME headers are defined in RFC 2045, which is part of the IETF.

At the beginning of email transmission, the server inserts the MIME header. A client decides which player application to use for a given data type based on the MIME header. Browsers and some web clients have built-in players. For example, all browsers support GIF and JPEG images and HTML files. It may be necessary to download other players like Installing Adobe Flash Player if they get an error of “No video with supported format and MIME type found” while trying to stream video on the Firefox browsers.

MIME converts non-ASCII data into 7 bits of Network Virtual Terminal (NVT) at the sender's end. NVT represents 128 characters in 7-bit ASCII. After MIME converts this data, the data is sent to the client via SMTP. It is then transferred to the recipient's end to see the contents regardless of whether they contain text, audio, video, or another kind of data.

mime working

Features of MIME 

Here are some features of the MIME Protocol. 

  • Multiple files can be sent in a single message this way.
  • Messages can be as long as you like.
  • Several binary attachments are available, such as images, executables, videos, audios, etc. All can be separated if need be.
  • There are a variety of content types and multipart messages supported by MIME.

Purpose and Functionality of MIME

MIME was mainly designed to solve the limitations of the early mailing systems. These systems were only able to support plain text at that time. MIME focuses on enabling the exchange of more diverse and complex data, like non-ASCII characters, through emails and other protocols.

The functionality of MIME also revolves around the exchange of diverse data, like images, audio, videos, etc., through emails and other internet protocols. MIME allows emails to include various types of non-text data like images, audio, videos, etc, and hence increases the range of content that can be sent through emails. MIME defines a structured way to differentiate data using subtypes such as “text/plain”, “image/jpeg,” etc. This helps the receivers to understand and interpret the attachments in the email.

MIME with SMTP and POP

SMTP is responsible for sending email messages, while POP is used to retrieve email messages from a server.

When an email message is composed of non-text data, MIME is used to encode the data into a text format that can be transmitted via SMTP. The email message is then sent using SMTP to the recipient's email server.

When the recipient retrieves the email message using POP, the email client reads the message header to determine the type of data included in the message using the Content-Type field. The email client then uses this information to properly display or handle the non-text data included in the email message.

MIME Header Fields

There are five headers that we add to the original header, Which are:

  • MIME-Version
  • Content-Type
  • Content-Type Encoding
  • Content Id
  • Content description


MIME-Version headers indicate whether a message is in MIME format. Here is the format of this header.

MIME-Version: 1.1


This Header file in MIME indicates the type of content included in the email. This can include emails, text, images, audios, videos etc. Some examples of possible content types are described below in a tabular format.

Example of Content type:-

TextPlain or HTML
MessageRFC822, Partial, External-Body
MultipartMixed, Parallel, Digest, Alternative
ApplicationPostScript, Octet-Stream


Content-Type Encoding

This header file indicates the encoding method used in the protocol to convert the binary data into a suitable format that can be transmitted via email. One common example of encoding is the Base64 encoding type.

Content Id

To allow email messages to be uniquely identified, this field appends a "Content Id" number.

Content-id: id=<content-id>

Content description

The content of the email is summarized in this field. This means that whatever is sent in the mail has information about it clearly in the "Content-Description."


Advantages of MIME Protocol

Here are some advantages of the MIME protocol:

  • Enables transmission of non-text data over email, allowing for more versatile messages.
  • Supports a variety of data types, including images, audio, and video.
  • Ensures that data is properly encoded and transmitted regardless of the recipient's email client.
  • Allows for the use of multiple parts in an email message, allowing for more complex messages to be sent.
  • Provides a standardized way to handle email attachments.

Frequently Asked Questions

What is a MIME type?

The MIME type is a system that differentiates the labels files based on the content they carry. This helps the client's system software to recognize them and easily handle and display them. A MIME type is generally made up as a combination of a primary type and a sub type.

What are the principles of MIME?

MIME works on the principles of content diversity, character encoding, structured data, secure transmission, modularity, compatibility etc. MIME protocol helps in versatile data exchange, easy interpretation by the user, safe transmission allows multi-part transmission, and ensures cross-system compatibility.

What is the MIME format for email?

The MIME format is used in emails to support multimedia and non-ASCII data types for transmission. The MIME format is a standard framework that enables the user to attach multiple data types like text, images, audio, etc., within emails using headers like Content-type, content-disposition, content-type-encoding, etc.

What is the port number for MIME?

The mail servers use TCP port 25 for communication, while mail clients use ports 587 or 465.04. The protocol is a TCP/IP application layer protocol.

What does the MIME boundary mean?

A MIME message consists of multiple message parts. MIME boundaries define the separation between message parts. In multi-part messages which contain multiple data types like images and audio, the MIME boundary acts as a marker to indicate the ending of the previous part and the start of the next part.

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Let's summarize what we have learned so far in this blog. First, we read that by using MIME, the capabilities of Internet email protocols such as SMTP are extended. Thanks to this protocol, users can send pictures, audio, video, and other types of files and documents in their emails. MIME should be included whenever non-ASCII characters are in a text, as it helps in the conversion.

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