ent to be read by the recipient. The header has: The sender address: Details of the address from which the message is sent. The receiver address: Details of the recipient address to which the message is to be sent. Message subject: An identifier for the message. The Internet user having an e-mail ID receives the mail to his mailbox that is periodically checked by the e-mail system and informs the user with a notice. If the Fundamentals of EMS, NMS and OSS/BSS user is ready to read his mail, a list containing the information about the mail like the sender mail address, the subject, the time when the mail was sent or received are displayed. The user can select the message of his choice and the contents of the message is displayed on the screen. E-mail addresses are required for delivering mail. The addressing system used by SMTP consists of the local part and the domain name that are separated by an @ symbol (local_identification@domain_name) Some of the terminologies that the reader will encounter in this chapter are: User agent: The user mailbox stores the received mails for a user for retrieval by the user agent. Email address: The name of the user mailbox is defined in the local part of the email address. Mail exchangers: These are the hosts that receive and send e-mail. The domain name assigned to each mail exchanger may be a DNS database or a logical name. This chapter has details on the user agent, mail transfer agents, and the mail delivery. SMTP is used for explaining the concepts. Perl programming language is used to show a programming example on implementing mail communication. Standard SMTP libraries are available in most of the high level programming languages. These libraries make client and server programming for mail communi- cation easy. This chapter also uses an SMTP library in Perl programming language to code the example program for mail communication. 29.2 Mail Delivery Process The SMTP is the TCP/IP protocol that defines e-mail services on the Internet. The client-server is used for exchange o