EMAIL (electronic mail - SMTP , ESMTP , MIME)

Email is the most widely used application service which is mostly used by computer users. It slightly differs from other uses of the networks as network protocols send packets directly to destinations using timeout and retransmission for individual segments if no ack returns. Yet in the case of email the system have to   provide  for  instances when the remote machine or the network connection has failed and take some particular action.Email applications involve two aspects -

• User-agent( pine, elm etc.)
• Transfer agent( sendmail daemon etc.)

When an email is sent it is the mail transfer agent (MTA) of the source that contacts the MTA of the destination end. The protocol used by the MTA 's on the source and destination side is known as SMTP. Here, full of SMTP is  Simple Mail Transfer Protocol.. There are a few protocols that come between the user agent and the MTA eg. POP,IMAP which are will be discussed later.

Mail Gateways -

Mail gateways are also known as mail relays, mail bridges and in such systems the senders machine does not contact the receiver's machine directly however, sends mail across one or more intermediate machines that forward it on. These intermediate machines are known as mail gateways.Mail gateways are brings unreliablity.Once the sender sends to 1st intermediate m/c then it discards its local copy. Therefore failure at an intermediate machine can result in message loss without informing the sender or the receiver. Mail gateways also bring delays. Neither the sender nor the receiver could determine  how long the delay will last or where it has been delayed.

On the other hand mail gateways have an advantage  providing interoperability ie. they offer connections among standard TCP/IP mail systems and other mail systems as well as among TCP/IP internets and networks that do not support Internet protocols. As a result when there is a change in protocol then the mail gateway helps in translating the mail message from one protocol to another as it will be designed to understand both.

SIMPLE MAIL TRANSFER PROTOCOL(SMTP)

TCP/IP protocol suite specifies a standard for the exchange of mail among machines. It was derived  from the (MTP )  Mail Transfer Protocol.  It have to deals with how the underlying  mail delivery system passes messages across a connection from one machine to another. The mail is enclosed in what is known as envelope . The enveilope contains the To and From fields and these are followed by the mail . The mail is having of two parts namely the Header and the Data.

The Header has the To and From fields.Now, If Headers are defined by us they should start with X. The standard headers do not start with the X. In SMTP data portion could contain only printable ASCII characters The old process of sending a binary file was to send it in uuencoded form but there was no way to distinguish among the many types of binary files these are .tar,  .gz , .dvi a lot more.

MIME(Multipurpose Internet Mail Extension)

This alllows the transmission of Non ASCII data througfh email, MIME allows random data to be encoded in ASCII and sent in a standard email message. every MIME message includes information that tells the recipient the kind of data and the sort of encoding used and this information in adding with the MIME version resides in the MIME header. Usually MIME header looks like this

MIME-Version: 1.0

Content-Description:

Content-Id:

Content-Type: image/gif

Content-Transfer-Encoding: base64

Content Descirption : contains the file name of the file that is being sent. Content -Type: is an vital field that specifies the data format ie. tells what type of data is being sent. It contains 2 identifiers a content type and a subtype separated by a slash. for example image/gif There are seven Content Types -

1.  text

2.  image

3.  video

4.  audio

5.  application

6.  multipart

7.  message

Content type - Message

It supports three subtypes namely

1.  RFC822 - the old mail message format

2.  Partial- means that ordinary message is just a part and the receiver should wait for all the parts before putting it in the mailbox.

3.  external_body - destination MTA will fetch file from remote site.

Content Type - Multipart

Multiple messages which may have different content types can be sent together. It supports four subtypes namely

1.  mixed -Look at each part independently

2.  alternative - The same message is sent in multiple types and formats and the receiver may choose to read the message in any form he wishes.

3.  parallel -The different parts of the message have to be read in parallel.That is,audio, video and text need to be read in a synchronised fashion

4.  digest -There are multiple RFC messages in mail. In this,The addresses of the receivers are in the form of a mailing list. Although file header is long it prevents cluttering of mail box.

PROBLEMS WITH SMTP

1.  There is no convenient  way to send nonprintable characters

2.  There is no way to know if one has received mail or not or has read it or not.

3.  Someone else can send a mail on my behalf.

So a better protocol was proposed - ESMTP ESMTP stands for Extended Simple Mail Transfer Protocol. It is well-suited with SMTP. Just as the first packet sent in SMTP is HELO similarly in ESMTP the first packet is called EHELO. Here,if the receiver supports ESMTP then it will answer to this EHELO packet by sending what data type and what kind of encoding it supports. Even a SMTP based receiver could reply to it. As well if there is an error message or there is no answer then the sender uses SMTP.

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