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.