What is an SMTP server? The SMTP server addresses we often use for emails are: QQ mailbox, Alibaba Cloud mailbox, Google mailbox, Sina mailbox. The so-called SMTP server is the SMTP server responsible for sending emails, that is, the Simple Mail Transfer Protocol. It is a set of rules for transmitting emails from the source address to the destination address, which controls the way the letter is transferred.
　　The SMTP protocol belongs to the TCP/IP protocol family, which helps each computer find the next destination when sending or transferring letters. From the transmission process of email, it can be seen that a fully functional email system should consist of two servers: the receiving server (POP3) and the sending server (SMTP). These two roles can be played by the same computer or by different computers.
　　1. The difference between SMTP server and IMAP or POP:
　　SMTP server is required to send emails, and POP server and IMAP server are required to receive emails. SMTP server is required to send (deliver) emails. When a user uses email software to send emails, the SMTP server set by the email software will send the email to the target server. Specifically, the mail software requests the SMTP server to deliver the email, and the SMTP server requesting the email delivery sends the outgoing email to the SMTP server that manages the target email address.
　　The role of the POP server: The POP server is an SMTP server required for sending mail. The POP server is required for receiving mail. The SMTP server sends mail to another party, and the POP server stores mail in the mail server.
　　IMAP server role: Like the POP server, the IMAP server is also a server required for receiving mail. For the POP server that downloads received mail to the recipient's personal computer, the mail is read by using the IMAP server.
　　When receiving mail, users will use a POP server or an IMAP server. The difference between the two is that the POP server downloads mail from the mail server to the recipient's computer, etc., while the IMAP server allows mail to be read while it is saved. IMAP servers are often used for reading.
　　What is an SMTP server?
　　II. How to let the SMTP server forward mail?
　　The SMTP service that comes with Windows Server 2003 can forward emails for anonymous users by default, but the destination of the email must be the domain it is responsible for.
　　If you want users to connect to the SMTP server using user accounts and passwords, the setting steps are as follows.
　　1. In the SMTP server, select [Start] → [Administrative Tools] → [Internet Information Services (IIS) Manager] command to open the IIS Manager console, expand the control tree in the left pane, right-click the [Default SMTP Virtual Server] icon, and select the [Properties] command in the pop-up shortcut menu.
　　2. In the [Default SMTP Virtual Server Properties] dialog box, select the [Access] tab.
　　3. In the [Authentication] dialog box, select the [Basic Authentication] or [Integrated Windows Authentication] check box, select the latter here. After setting, click the [Confirm] button, return to the [Default SMTP Virtual Server Properties] dialog box, and click the [Confirm] button again.
　　4. Modify the settings of the mail client Outlook Express. In the mail client, start Outlook Express, open the Internet Account Properties dialog box, select the [Server] tab, select the [My server requires authentication] check box, and then click the [Set] button.
　　5. In the [Send Mail Server] dialog box, select the [Login Method] radio button and enter the account name and password for logging into the SMTP server. It should be noted that the account name here does not need the attachment domain name. For example, if the email account is xxxx@.com, you only need to enter "xxxx". After setting, click the [OK] button.
　　IP address and TCP port number settings: If the computer where the SMTP service is installed has multiple IP addresses, the user can select the IP address that provides the SMTP service. In this way, the client sends an email to the SMTP server through the selected IP address, and the SMTP server will receive and forward the email. The port number is used to set different services in the computer. The default port number of the SMTP service is 25. Generally, do not change the port number at will.
　　SMTP server connection settings
　　Connection settings: When a user or other SMTP server sends an email to the local SMTP server, a TCP connection is established between the two, or when the local SMTP server forwards an email to another SMTP server, a TCP connection is also established between the two. For the local SMTP server, the former is called an incoming connection and the latter is called an outbound connection. The administrator can limit the number of incoming or outgoing connections to make it more difficult for network hackers to attack the SMTP server.
　　Message delivery settings: The functions mainly include retransmission and interval time settings, email jump point settings, masquerade domain settings, fully standardized domain name settings, interrupt host settings and reverse DNS query settings.
　　Mail management settings mainly include connection restrictions, email size restrictions, recipient number restrictions and how to handle undeliverable emails. These can be set through the [Delivery] tab of the [Default SMTP Virtual Service Properties] dialog box