Email Protocols - Internet Technologies

What are Email protocols?

E-mail protocols are the rules for transmitting the data or information from client to the mail server. We shall study more about such protocols such as SMTP, POP and IMAP.

SMTP

SMTP abbreviates Simple Mail Transfer Protocol used in sending e-mail reliably and efficiently over the internet. It is first proposed in 1982.

Key Points:

  • SMTP is application level, connection oriented and text based protocol.
  • Messages are exchanged between e-mail servers over TCP/IP network.
  • Notification is also offered by SMTP once the e-mail is sent.
  • SMTP client is used to coordinate the email server and the recipient mail server when any email is sent.
  • SMTP commands designate the sender’s and receiver’s e-mail address with message.
  • No intervention by any user while the commands are exchanged between servers.
  • SMTP sends the error report if any message is not delivered making it a reliable protocol.

SMTP Commands

Below are the SMTP commands.

S.N.

Command Description

1

HELLO

This command initiates the SMTP conversation.

2

EHELLO

This is an alternative command to initiate the conversation. ESMTP indicates that the sender server wants to use extended SMTP protocol.

3

MAIL FROM

This indicates the sender’s address.

4

RCPT TO

It identifies the recipient of the mail. In order to deliver similar message to multiple users this command can be repeated multiple times.

5

SIZE

This command let the server know the size of attached message in bytes.

6

DATA

The DATA command signifies that a stream of data will follow. Here stream of data refers to the body of the message.

7

QUIT

This commands is used to terminate the SMTP connection.

8

VERFY

This command is used by the receiving server in order to verify whether the given username is valid or not.

9

EXPN

It is same as VRFY, except it will list all the users name when it used with a distribution list.

IMAP

IMAP abbreviates for Internet Mail Access Protocol, first proposed in 1986. The IMAP versions are below.

Original IMAP

  • IMAP2
  • IMAP3
  • IMAP2bis
  • IMAP4

Key Points:

  • One can operate the e-mail message on the server even without downloading it on the local using IMAP.
  • Remote server holds and maintains the e-mail.
  • IMAP allows us to download, delete the mail without reading the mail. It offers us to create, manipulate and delete remote message folders called mail boxes.
  • IMAP offers the users to search for the e-mails.
  • Multiple mailboxes are accessed on multiple mail servers.

IMAP Commands

Below table has some of the IMAP commands:

S.N.

Command Description

1

IMAP_LOGIN

This command opens the connection.

2

CAPABILITY

This command requests for listing the capabilities that the server supports.

3

NOOP

This command is used as a periodic poll for new messages or message status updates during a period of inactivity.

4

SELECT

This command helps to select a mailbox to access the messages.

5

EXAMINE

It is same as SELECT command except no change to the mailbox is permitted.

6

CREATE

It is used to create mailbox with a specified name.

7

DELETE

It is used to permanently delete a mailbox with a given name.

8

CHANGE

It is used to change the name of a mailbox.

9

LOGOUT

This command informs the server that client is done with the session. The server must send BYE untagged response before the OK response and then close the network connection.

POP

POP abbreviates for Post Office Protocol used to support a single client. POP3 is the current version in use.

Key Points

  • It is an application layer internet standard protocol.
  • POP takes less internet usage time as it supports offline access to the messages.
  • Searching is not allowed in POP.
  • One has to download the messages in order to access.
  • Only one mail box is created on server.
  • POP not offers accessing non mail data.
  • POP commands are generally abbreviated into codes of three or four letters. Eg. STAT.

POP Commands

Below table explains some of the POP commands:

S.N.

Command Description

1

LOGIN

This command opens the connection.

2

STAT

It is used to display number of messages currently in the mailbox.

3

LIST

It is used to get the summary of messages where each message summary is shown.

4

RETR

This command helps to select a mailbox to access the messages.

5

DELE

It is used to delete a message.

6

RSET

It is used to reset the session to its initial state.

7

QUIT

It is used to log off the session.

Comparison between POP and IMAP

S.N.

POP

IMAP

1

Generally used to support single client.

Designed to handle multiple clients.

2

Messages are accessed offline.

Messages are accessed online although it also supports offline mode.

3

POP does not allow search facility.

It offers ability to search emails.

4

All the messages have to be downloaded.

It allows selective transfer of messages to the client.

5

Only one mailbox can be created on the server.

Multiple mailboxes can be created on the server.

6

Not suitable for accessing non-mail data.

Suitable for accessing non-mail data i.e. attachment.

7

POP commands are generally abbreviated into codes of three or four letters. Eg. STAT.

IMAP commands are not abbreviated, they are full. Eg. STATUS.

8

It requires minimum use of server resources.

Clients are totally dependent on server.

9

Mails once downloaded cannot be accessed from some other location.

Allows mails to be accessed from multiple locations.

10

The e-mails are not downloaded automatically.

Users can view the headings and sender of e-mails and then decide to download.

10

POP requires less internet usage time.

IMAP requires more internet usage time.

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