E mail protocol - SMTP

CS450 – Internet Protocol

Md Syed Ahamad 1301030

CS450 - Internet Protocol presentation

1

Introduction Fundamentals

SMTP model and protocols End-to-end mailing system Stored-and-forward mailing system SMTP mail transaction flow

Mail Access modes Mail Access protocols

CS450 - Internet Protocol presentation 2

Most email systems that send mail over the internet use SMTP to send messages from server to another server.

Email client retrieves the messages using POP & IMAP.

SMTP works in client – server architecture between mail servers.


SMTP is used as the common mechanism for transporting email among different host within TCP/IP suite. It is application layer protocol at port 25. Mail arrived via SMTP is forwarded to a remote server. POP3 and IMAP allows user to download from local

server. Email clients like Eudora allow the client to specify an SMTP

server and POP server.

On UNIX based system, Sendmail is widely used SMTP server. It includes POP3 and also comes in a version for Windows NT


User Agent(UA): It is a program that is used to send/receive email. This are email clients. Unix based - Berkeley mail, Elm, MH, Pine etc. Windows based - MS outlook, Netscape/Mozilla Communication

Message Transfer Agent(MTA) - transfers email messages from one computer to another using a client–server application architecture. MTA maintains queue for scheduling repeat delivery attempts in case a remote server is

unable. Unix based – Sendmail, eMailman, etc. Windows based – MS Exchange.


Model:

End-to-end mail delivery: used between organisations No intermediate message transfer agents (MTAs). SMTP client keeps the mail items until it is successfully transferred to

recipient/destination.



Stored-and-forward mail delivery method: It works within a network or organisation that have TCP/IP and SMTP based networks. Mail item may pass through a number of intermediate hosts in the same network on its

way to destination. Successful transmission indicates that the mail has reached the neighbour intermediate

hop.



Few cases of sending email across network: Message – sender prepares messages -> creates envelope -> put the messages in

envelope -> MTA transfers mail to TCP/IP port 25 of receiver MTA. Relay MTA could be involved where the system may or may not be on TCP/IP suite. Email gateway or mail bridges – exchange capability of TCP/IP SMTP and locally used

mailing systems. It can receive an email prepared by a protocol other than SMTP and transform it to the SMTP

format before sending it. It can also convert SMTP format to another format and send it to the destination which does not use

SMTP TCP/IP suite. It may alter end-to-end delivery specification. Behaviour is defined by end-to-end transmission at host-to-gateway, gateway-to-host, gateway-to-

gateway.


Suppose [email protected] sends email to [email protected], [email protected], and [email protected].

Sending email: Message – body (data). Envelope (header) is transmitted separately from message itself Sender’s host – sun.it.revier.edu Receiver’s host - mail.unh.edu @ is used to represents several email addresses – one to many alias expansion. jsmith, byan are the computer reachable address of the mailboxes. SMTP uses DNS to determine IP address of the destination mailbox.

Receiving Email: UA periodically checks mailboxes about the mail arrival.


mailto:[email protected]






SMTP destination address user@host – TCP/IP User%remote-host@gateway-host – non SMTP destination host via gateway-host. @host-a,@host-b:user@host-c – for relay messages (explicit routing information) Not eligible for end-to-end delivery system and it is no longer used.

Delay messages: messages can be delay at sender site, receiver site or the intermediate servers. Sender site – It will be storing in spooling system and client mail periodically (10 to 15

minutes) checks the spool to find the mail can be sent. If it can’t be delivered in the timeout period (3 to 5 days), the mail returns to the sender.

Intermediate MTA also has mail storage and spooling storage.




A mail client (UA) is the software that a user employs to read, send, file, and otherwise process the electronic mail. The mail client also manages related e-mail data (address books, spelling dictionaries, and

stationery). The mail client connects to a mail server to retrieve new mail. The communication between the mail client and mail server is regulated by the mail access protocol,

a standardized set of transmitted commands and responses sent over many different types of network connections

A mail access protocol operates in three common modes Offline mode - e-mail is downloaded from a temporary storage on the mail server to the user’s

computer. After download, the mail is deleted from the server.

Online mode – permanent storage at mail server but temporary storage at client mailbox.

Disconnected/resynchronization mode – combines both offline and online mode.


POP3 – It can be used with or without SMTP. POP2 was designed for SMTP only. Offline , Resynchronized modes supported. It allows users to download only header to one season. No interoperability issues End user mailbox at server mailbox for end-to-end users.

IMAP4 - a protocol for retrieving email. Used for remote access in the mail server. It supports additional features over POP i.e. search message on mail server. IMAP is like post office and SMTP as postal carriers. Used TCP/IP for reliability.


Vladimir V. Riabov, SMTP ((Simple Mail Transfer Protocol)), Revier College.

Rfc – 5321.
