Electronic Mail

One  of the  most popular internet  services is electronic mail ( e mail). At  the beginning  of the internet era teh message sent by  electronic  mail were short and  consisted of text  only they let people  exchange  quick  memos. Today  electronic  mail is such  more  complex. It allows  a message  to include  text audio  and video  it also  allows one message to sent  to one or more  recipients.

Figure presents a high  level  view of the internet  of the  internet mail  system. It has  three major components user agents mail servers, and  the simple  mail  transfer protocol (SMTP). We now  assume  that a  sender  heena sending an e mail  message  to a recipient Harish. User  agents allow users to read reply to forward save and compose message. When  server when Harish  wants to read a message his user agent retrieves the message from his mailbox in his mail server.

                                                                figure A high  level view of the Internet e- mail system

Each recipient  such as harsh  has a mailbox  located in one of the servers. Harish  mail box manages  and maintains the messages that have been sent to him. A  messages  starts its journey in the sender user agent  travels to the sender mail server and travels  to the recipient mail  server where  it is deposited in the recipient mail box  when  box  authenticates Harish  ( with  user name and password).  If Heena serve cannot  deliver mail to  Harish  server Heena  server holds the message in a message  queue  and attempts to transfer the message  later.