Page 192 - DCAP406_DCAP_207_Computer Networks

Unit 12: Application Layer

3. MTA needs to deliver copies of a message to several machines. Notes
4. MTA has to allow mixing text, voice, and video in a message and appending documents
and files to a message.

As discussed above, the e-mail addresses consist of the following components:
Mailbox names: A mailbox is associated with one login id within a mail server to store the
e-mails of the user. Therefore, a specific name is provided to the mailbox associated with each
IDs.
Symbolic names: It refers to the name of a service rather than a specific user. For example,
postmaster is universally recognized as an address for post mail problems. In e-mail system, the
symbolic names are aliases for specific mailboxes.
Group names (mail exploders): It refers to an alias for a set of recipients. MTA consults an
internal database to specify the mail addresses.
There are number of e-mail packages available. Some of them are free like Goggle’s mail, Yahoo
mail, hotmail etc while some are paid. All of them are also not alike but most of the e-mail
software has some basic functionality common. These are:

Send and receive mail messages
Save your messages in a file
Print mail messages
Forward a mail message to other recipient

Reply to mail messages
Attach a file to a mail message
In order to send a message we need to first type the address of the intended recipient.
E-mail addresses have some sort of similarity with phone numbers with regard to the
identification of person, organization, or a geographic location. E-mail addresses likewise,
telephone numbers, which have usually area code, have rules for use. Usually, the e-mail address
has three parts:
1. A user identity or name
2. An “at” sign (@)

3. The domain name, which basically specifies the address of the user’s mail server. It is the
right most part of the address and follow a particular naming conventions. You can now
understand the e-mail address by the help of the following example:-
Example - services@jalandhar.in
The left most part before the @ (at sign) is the identity or name if the user and the right most part
after the @ is the server which indicates India. There are some naming conventions like edu,
com, org etc used for education, commercial, organization respectively.
The Simple Mail Transfer Protocol is the de facto standard an electronic mail (e-mail) service
provider. It is intended for the transfer of e-mail messages across the network. The protocol
itself is simple because it uses the services of TCP where much of the hard work is handled by
lower-level protocols. SMTP uses TCP transport for the reliable delivery of mail messages. For
this purpose MTA opens a TCP connection to a destination location and sends the message to the
destination at this location. The remote MTA at the mail server of remote location stores the
message in its storage and returns an acknowledgment after it has saved the message successfully.
Thereafter, the sender removes its copy. When the destination address is unavailable, the MTA

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