The Differences and Relationships between POP3, SMTP and IMAP

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The Differences and Relationships between POP3, SMTP and IMAP [Copy link]

1. Introduction to POP3 POP3, full name is "Post Office Protocol - Version 3", that is, "Post Office Protocol Version 3". It is a member of the TCP/IP protocol family and is defined by RFC1939. This protocol is mainly used to support the use of clients to remotely manage emails on the server. The POP3 protocol that provides SSL encryption is called POP3S. The POP protocol supports "offline" mail processing. The specific process is: the mail is sent to the server, and the email client calls the mail client program to connect to the server and download all unread emails. This offline access mode is a store-and-forward service that sends mail from the mail server to a personal terminal machine, usually a PC or MAC. Once the mail is sent to the PC or MAC, the mail on the mail server will be deleted. However, most of the current POP3 mail servers can "only download mail, and the server does not delete it", which is an improved POP3 protocol. POP3 Operation Guide: The server allows mail clients that comply with POP3 (PostOfficeProtocol, Version 3 Mail Delivery Protocol, Version 3) to connect to the Imail server. These mail clients include Outlook Express, Outlook, Netscape Messenger or Communicator, Eudora, Pegasus, NuPOP, Z-Mail, FoxMail, TheBat, Kmail, and Unixmail [2]. POP3 clients usually use the "off-line" offline method to access the mail server. They will access the mail server at regular intervals, download mail to the client's computer, and then disconnect from the server. Generally, the mail is temporarily stored on the server. When the client downloads these mails, they will be deleted by the server and no longer retained. This method is very suitable for users who always read mail on the same computer. Another method, called the "online" online method, is that the mail client always maintains a connection with the server. Mail is kept on the server, the client does not download mail to the client, and the user can read the mail kept on the server online. This method is suitable for users who often use different computers. ImailPOP3 service can be run as a Windows NT service, completely hidden or in a certain interactive way. The service will always run even if you log out of the system. IMail also provides another access method IMAP4 (InternetMessageAccessProtocolVersion4). IMAP4 service provides both "online" and "offline" access methods. Logging On Login By default, the POP3 service uses the system account to log in to the system. You can change this login method: 1. In the control panel, start the Services applet. 2. Select the IMailPOP3 Server service and click Startup. 3. (Optional) Select the StartupType you want. 4. In Log On As, select the correct login account: IMailServerSystemAccount. If your mail host uses the Windows NT user database as the Imail user database, then you need to use this account. AllowServicetoInteractwithDesktop. When this account is used, there will be an Imail icon on the desktop when any user logs into the system. ThisAccount.When using this method, if your host does not use the Windows NT user database, you can enter the user account you want; make sure the account is a host administrator. If your mail host uses an external database, make sure the account can access the external database. Configuring the POP3 Server How to configure the POP3 server: 1. In IMail Administrator, select "localhost" in the left panel. 2. Select the "Services" directory and click POP3.The POP3 panel appears. 3. Enter the options described below to configure the POP3 service. 4. Click the Apply button to save your settings. 5. Stop the service, wait 5-10 seconds and then restart the service. When you click the Stop/Start button, the changes to the settings will be saved automatically. Understand the differences and connections between POP3, SMTP and IMAP in one article 2. Introduction to SMTP SMTP (Simple Mail Transfer Protocol) is a set of rules for transmitting mail from a source address to a destination address, which controls the way letters are transferred. The SMTP protocol belongs to the TCP/IP protocol suite, which helps each computer find the next destination when sending or forwarding letters. Through the server specified by the SMTP protocol, the E-mail can be sent to the recipient's server, and the whole process only takes a few minutes. The SMTP server is a sending mail server that follows the SMTP protocol and is used to send or forward outgoing emails. It uses the reliable data transmission service provided by TCP to transfer email messages from the sender's mail server to the recipient's mail server. Like most application layer protocols, SMTP also has two ends: the client executed on the sender's mail server and the server executed on the recipient's mail server. The SMTP client and server run on each mail server at the same time. When a mail serverWhen a server sends mail messages to other mail servers, it runs as an SMTP client. SMTP working process: Simple Mail Transfer Protocol (SMTP) is a text-based email transmission protocol, which is used to exchange mail between mail servers on the Internet. SMTP is an application layer service that can be adapted to various network systems. SMTP commands and responses are text-based, with command lines as units and line breaks as CR/LF. The response message is usually only one line long, starting with a 3-digit code, followed by a very brief text description. SMTP goes through three stages: establishing a connection, sending mail, and releasing the connection. Specifically: (1) Establishing a TCP connection. (2) The client sends a HELO command to the server to identify itself, and then sends a MAIL command. (3) The server responds with OK, indicating that it is ready to receive. (4) The client sends an RCPT command. (5) The server indicates whether it is willing to receive mail for the recipient. (6) The negotiation is over, and the mail is sent. The input content is sent using the DATA command. (7) The sending is finished and the QUIT command is used to exit. SMTP server routes email based on the mail exchange (MX) record in DNS. When sending email, the email system locates the mail server according to the recipient's address suffix. SMTP completes the functions of editing, receiving and reading emails through the user agent (UA); and transmits emails to the destination through the mail transfer agent (MTA). Understand the differences and connections between POP3, SMTP and IMAP in one article 3. Introduction to IMAP IMAP (Internet Mail Access Protocol) was formerly known as Interactive Mail Access Protocol. IMAP is a mail retrieval protocol developed by Stanford University in 1986. Its main function is to enable mail clients (such as MS Outlook) to access mail messages.Express) can obtain email information from the mail server, download emails, etc. through this protocol. The current authoritative definition is RFC3501. The IMAP protocol runs on top of the TCP/IP protocol and uses port 143. The main difference between it and the POP3 protocol is that users do not need to download all emails, but can directly operate on the emails on the server through the client. Features of imap: Similar to the POP3 protocol, IMAP (Internet Message Access Protocol) also provides user-oriented email collection services. The commonly used version is IMAP4. IMAP4 improves the shortcomings of POP3. Users can decide whether to collect, delete and retrieve specific parts of emails by browsing the headers of emails. They can also create or change folders or mailboxes on the server. In addition to supporting the offline operation mode of the POP3 protocol, it also supports online operation and disconnected operation. It provides users with the function of selectively receiving emails from the mail server, server-based information processing functions and shared mailbox functions. The offline mode of IMAP4 is different from POP3. It will not automatically delete the emails that have been taken out from the mail server. Its online mode and disconnected mode also access the mail server as a "remote file server", which is more flexible and convenient. IMAP4 supports multiple mailboxes. These features of IMAP4 are very suitable for users who operate emails between different computers or terminals (for example, you can operate the same mailbox through the email agent on your mobile phone, PAD, and PC), as well as those who use multiple mailboxes at the same time. imap function: Supports two operation modes: connect and disconnect. When using POP3, the client will only connect to the server for a period of time until it has downloaded all new information, and then the client will disconnect. In IMAP, the client remains connected to the server as long as the user interface is active and downloading the message content is needed. For users with many or large emails, using IMAP4 mode can achieve faster response times. Support multiple clients connecting to a mailbox at the same time. The POP3 protocol assumes that the current connection to the mailbox is the only connection. In contrast, the IMAP4 protocol allows multiple users to access the mailbox at the same time and provides a mechanism for clients to be aware of the actions of other users currently connected to the mailbox. Support for accessing MIME parts of messages and partial retrieval. Almost all Internet email is transmitted in MIME format. MIME allows messages to contain a tree structure with leaf nodes of a single content type rather than leaf nodes that are combinations of multiple types. The IMAP4 protocol allows clients to retrieve any individual MIME part and retrieve part or all of the message. These mechanisms allow users to browse the message content without downloading attachments or browse while retrieving the content. Support for retaining message status information on the server. By using flags defined in the IMAP4 protocol, clients can track message status, such as whether a message has been read, replied to, or deleted. These flags are stored on the server, so that multiple clients accessing a mailbox at different times can be aware of the actions taken by other users. Support for access to multiple mailboxes on a server. IMAP4 clients can create, rename, or delete mailboxes on the server (usually presented to the user as folders). Support for multiple mailboxes also allows the server to provide access to shared and public folders. Support for server-side searching. IMAP4 provides a mechanism for clients to ask the server to search for information that meets multiple criteria. This mechanism does not require the client to download all the information in the mailbox to complete these searches. Support for a well-defined extension mechanism. Drawing on the experience of early Internet protocols, IMAP extensions define a clear mechanism. Many extensions to the original protocol have been proposed and are widely used. Whether using POP3 or IMAP4 to get messages, the client uses the SMTP protocol to send them. Mail clients may be POP clients or IMAP clients, but they all use SMTP. Understand the differences and connections between POP3, SMTP and IMAP in one article 4. The differences and connections between POP3, SMTP and IMAP POP3 is an electronic protocol that specifies how to connect a personal computer to an Internet mail server and download emails. It is the first offline protocol standard for Internet email. POP3 allows users to store emails from the server to the local host (that is, their own computer) and delete emails stored on the mail server. The POP3 server is a receiving mail server that follows the POP3 protocol and is used to receive emails. The POP3 protocol allows email clients to download emails from the server, but operations on the client (such as moving emails, marking as read, etc.) will not be fed back to the server. For example, if 3 emails in the mailbox are collected through the client and moved to other folders, these emails on the mailbox server will not be moved at the same time.. IMAP provides two-way communication between webmail and email clients. The client's operations will be fed back to the server. The server will also take corresponding actions on the mails. At the same time, IMAP provides a convenient mail download service like POP3, allowing users to read offline. The summary browsing function provided by IMAP allows you to decide whether to download after reading all the information such as the arrival time, subject, sender, size, etc. of the mail. In addition, IMAP better supports accessing new mails from multiple different devices at any time. SMTP is a set of specifications for transmitting mail from the source address to the destination address, which controls the way the mail is transferred. The SMTP protocol belongs to the TCP/IP protocol cluster, which helps each computer find the next destination when sending or transferring letters. The SMTP server is a mail sending server that follows the SMTP protocol. SMTP authentication, in simple terms, requires that the account name and password must be provided before logging into the SMTP server, which makes it impossible for spammers to take advantage of it. The purpose of adding SMTP authentication is to prevent users from being disturbed by spam. IMAP stands for Internet Mail Access Protocol, which is an interactive mail access protocol. It is one of the standard mail access protocols similar to POP3. The difference is that after turning on IMAP, the mails you receive in the email client will still remain on the server, and the operations on the client will be fed back to the server, such as deleting mails, marking as read, etc., and the mails on the server will also take corresponding actions. So no matter you log in to your mailbox from a browser or client software, the mails and status you see are the same.