MS, NMS and OSS/BSS 29.3 Mail Protocols a. Simple mail transfer protocol (SMTP): This protocol pushes the messages to the receiver without the server requesting a transfer. So SMTP is a push- based protocol. It is used to send data from client to a server where the mails are stored. Some other access protocol is to be used by the recipient to retrieve the messages from the mail server mailbox where it was stored. b. Post office protocol, version 3 (POP3): This is a mail access protocol. The recipient computer has client POP3 software and the mail server has the server POP3 software. The client can access mail by downloading e-mail from the mail server mailbox. A connection is opened between the user agent or the client and the server on TCP port 110. The user name and password are sent for authentication. The delete mode and the keep mode are the two modes of POP3. After the retrieval of the mails, they are deleted from the mailbox through the delete mode. The mails are stored in the user's local workstation where they can save and organize the received mails after reading or replying. In the keep mode, the mails remain inside the mailbox even after retrieval and are employed. The keep mode is used when the user accesses the mails through a system away from the local workstation, SO that the mail can be read and maintained for later retrieval and organizing. However POP3 has several limitations. C. Internet mail access protocol, version 4 (IMAP4): It is a mail access proto- col. IMAP4 is a powerful and complex mail access protocol used to handle the transmission of email. Using IMAP4 the user can create the mailbox hierarchy in a folder for e-mail storage or even create, delete, or rename the mailboxes on the mail server. In cases where the bandwidth is low and the e-mail contains multimedia with high bandwidth requirements, the user can partially download the e-mail through IMAP4. It is also possible to check the e-mail header or search for a string of characters in the e-mail content before downloading. 29.4 Us