SMTP is a protocol used to maintain the transmission order and specify what should be done between mail servers. Its goal is to transmit e-mail reliably and effectively. SMTP is independent of the delivery subsystem and can relay messages.
It is unrealistic to maintain a message transmission system (MTS) on a relatively small node of the Internet. For example, the workstation may not have enough resources to allow the SMTP server and related local mail delivery system to stay and run continuously.
IMAP4 provides a means to manage mail on a remote mail server, which can provide users with functions such as selectively receiving mail from the mail server, processing information based on the server, and enjoying the mailbox.
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Characteristics of e-mail
E-mail is a system that directly faces the information exchange between people in the whole network and even all other network systems. Its data sender and receiver are both human beings, so it greatly meets the communication needs of a large number of people.
E-mail refers to the communication mode of transmitting letters, documents, materials and other information by electronic means. E-mail combines the characteristics of telephone communication and postal letters. It can transmit information as quickly as a telephone and make the receiver receive a written record like a letter at the receiving end. E-mail system is also called computer-based mail information system.