History The first e-mail message was sent in 1971 by an engineer named Ray Tomlinson.

1. History The first e-mail message was sent in 1971 by an engineer named Ray Tomlinson. Made possible by using the @ sign to designate the receiving machine. Email and Email Clients? Email is a piece of text Eg of Email clients – yahoo, Outlook, gmail. Functions of Email client. It shows you a list of all of the messageswith headers. Create select and read messages. Also work with attachments.

2. A Simple Email Server • Connects to the Email Client. • Any machine which provides services to other machines is a server. Egs of servers: Web Servers, Ftp Servers, Email Servers… • Listen at specific ports.

3. Working • Email Server has a list of accounts for each client • Each account has a text file. • When email client creates msg. • Domain name is striped off and that particular email server is contacted. • The email is added at the end of the appropriate text file.

4. When the recipient checks the email. The email client of the receiver copies the file and the server file is erased.

5. Real-Email System • Email Servers consists of SMTP (Simple Mail Transfer Protocol) server and the POP3 (Post Office Protocol) or IMAP (Internet Mail Access Protocol) server • The SMTP server handles outgoing mails.

6. The Email Client contacts the Email server to the SMTP server which is listening at Port 25. • SMTP server takes the “To” address and breaks it into 2 parts • The recipients name Eg: “jsmith” • The name domain name Eg: “Yahoo.com” • It checks if it belongs to its own domain (jsmith@abc.com to paul@abc.com ) If so it hands it to the POP3 server using a program called the “delivery agent” • POP3 server attaches the mail the text file of the “to” account holder. • What if the domain names of “from” and “to” are not the same? Eg: mail from jsmith@abc.com to mbrain@xyz.com

7. “abc” SMTP server of contacts DNS (Domain Name Server) to get the IP of the “xyz” domain. • DNS returns IP for that domain name. • SMTP of the “abc” contacts with the SMTP of the “xyz” through the IP returned by DNS.

8. The SMTP of “xyz” forwards to its POP3 server. • POP3 attaches the message to the text file of the account holder. • What if SMTP of “abc” cannot contact SMTP of “xyz”? • It goes into the queue of “abc” called the “sendmail queue” • Abc’s SMTP server retrys to resend it to xyz’s SMTP every 15 minutes. • After 4 hours if still not successful it return a message to the sender “Mail not delivered” Requests For Comments (RFC) • Conversations between the SMTP server and the email Client.

10. HELO - introduce yourself • EHLO - introduce yourself and request extended mode • MAIL FROM: - specify the sender • RCPT TO: - specify the recipient • DATA - specify the body of the message (To:, From: and Subject: should be the first three lines.) • RSET - reset • QUIT - quit the session • HELP - get help on commands • VRFY - verify an address • EXPN - expand an address • VERB - verbose

11. Thank Q. Questions?