SMTP, POP, POP3, IMAP, MIME

Simple Mail Transfer Protocol (SMTP):-

• Email is emerging as one of the most valuable services on the internet today. Most of the internet systems use SMTP as a method to transfer mail from one user to another. SMTP is a push protocol and is used to send the mail whereas POP (post office protocol) or IMAP (internet message access protocol) are used to retrieve those mails at the receiver’s side.  

(SMTP):-

• SMTP is an application layer protocol. The client who wants to send the mail opens a TCP connection to the SMTP server and then sends the mail across the connection. The SMTP server is always on listening mode. As soon as it listens for a TCP connection from any client, the SMTP process initiates a connection on that port (25). After successfully establishing the TCP connection the client process sends the mail instantly. 

SMTP Protocol:-

The SMTP model is of two type:

1. End-to-end method
2. Store-and-forward method

Model Of SMTP System:-

• In the SMTP model user deals with the user agent (UA) for example Microsoft Outlook, Netscape, Mozilla, etc. In order to exchange the mail using TCP, MTA is used. The users sending the mail do not have to deal with the MTA it is the responsibility of the system admin to set up the local MTA. The MTA maintains a small queue of mails so that it can schedule repeat delivery of mail in case the receiver is not available. The MTA delivers the mail to the mailboxes and the information can later be downloaded by the user agents. 

Both the SMTP-client and MSTP-server should have 2 components:-

1. User agent (UA)
2. Local MTA

Post Office Protocol (POP):-

• Post Office Protocol (POP) is a type of computer networking and Internet standard protocol that extracts and retrieves email from a remote mail server for access by the host machine. 

• POP is an application layer protocol in the OSI model that provides end users the ability to fetch and receive email. 

• Post Office Protocol is the primary protocol behind email communication. POP works through a supporting email software client that integrates POP for connecting to the remote email server and downloading email messages to the recipient’s computer machine. 

• POP3 stands for Post Office Protocol Version3 (Current Version). POP is a protocol which listens on port 110 and is responsible for accessing the mail service on a client machine. POP3 works in two modes such as Delete Mode and Keep Mode. 

1. Delete Mode: A mail is deleted from the mailbox after successful retrieval. 
2. Keep Mode: The Mail remains Intact in the mailbox after successful retrieval. 

Difference between SMTP and POP3:-

• For sending and receiving messages, we use two protocols one is SMTP (Simple Mail Transfer Protocol) and another is POP3 (Post Office Protocol version 3). They are also called as PUSH and POP protocols respectively. They are agents, Message Transfer Agent and Message Access Agent respectively to send and retrieve the messages. 

SMTP:-

1. The port number of SMTP is 25.
2. It is a MTA (Message Transfer Agent) for sending the to the receiver. 

POP3:-

1. The port number of POP3 is 110.
2. It is MAA (Message Access Agent) for accessing the messages from mail box. 
3. It has also two MAAs one is client MAA (Message Access Agent) and another is server is server (Message Access Agent). MAA
4. POP3 is also known as POP protocol. 

IMAP (Internet Message Access Protocol):-

• IMAP (Internet Message Access Protocol) is a standard email protocol that stores email messages on a mail server, but allows the end user to view and manipulate the messages as though they were stored locally on the end user's computing device(s). This allows users to organize messages into folders, have multiple client applications know which messages have been read, flag messages for urgency or follow-up and save draft messages on the server. 

• IMAP can be contrasted with another client/server email protocol, Post Office Protocol 3 (POP3). With POP3, mail is saved for the end user in a single mailbox on the server and moved to the end user's device when the mail client opens. While POP3 can be thought of as a "store-and-forward" service, IMAP can be thought of as a remote file server. 

Multipurpose Internet Mail Extension (MIME) Protocol:-

• Why do we need MIME?

       Limitations of Simple Mail Transfer Protocol (SMTP):

• SMTP has a very simple structure 
• It’s simplicity however comes with a price as it only send messages in NVT 7-bit ASCII format. 
• It cannot be used for languages that do not support 7-bit ASCII format such as- French, German, Russian, Chinese and Japanese, etc.so it cannot be transmitted using SMTP. So, in order to make SMTP more broad we use MIME. 
• It cannot be used to send binary files or video or audio data. 

Purpose and Functionality of MIME:-

• Growing demand for Email Message as people also want to express in terms of Multimedia. So, MIME another email application is introduced as it is not restricted to textual data. 

• MIME transforms non-ASCII data at sender side to NVT 7-bit data and delivers it to the client SMTP. The message at receiver side is transferred back to the original data. As well as we can send video and audio data using MIME as it transfers them also in 7-bit ASCII data. 

Features of MIME:-

• It is able to send multiple attachments with a single message. 
• Unlimited message length. 
• Binary attachments (executables, images, audio, or video files) which may be divided if needed. 
• MIME provided support for varying content types and multi-part messages.