In the world of the Internet, we send and receive emails every day, from work notifications, registration verification to greetings from friends, and there is a core support behind this - the mail server. It is the "brain" of email sending and receiving. Once it goes down or is misconfigured, our email system will be "paralyzed". So, what exactly is Mail Server? How does it work? This article will explain it in simple and straightforward language.

　　Mail Server is a server dedicated to handling email sending, receiving, transmission and storage. It is like a digital post office, managing the "sending" and "receiving" processes of emails.

　　Let's take a look at a simple example:

　　Scenario: You send an email to your friend using a client.

　　Step 1: The email client submits the email to the SMTP server

　　You click "Send", and the client first contacts the SMTP server you configured; this server will verify your identity (user name + password); after the verification is passed, it will "deliver" the email.

　　Step 2: The SMTP server looks up the MX record of the other party's domain name

　　It resolves the domain name in the other party's email address (such as example.com in friend@example.com); then it goes to DNS to look up the mail server address of example.com (i.e., the MX record); after finding it, it is like a postman finding the location of the other party's post office.

　　Step 3: Send the mail to the other party's SMTP receiving server

　　Then your SMTP server will send the mail to the other party's SMTP receiving end; the other party's server usually runs a Mail Transfer Agent (MTA); it receives the mail, determines whether the target address exists, and stores the mail content.

　　Step 4: The other party's user reads the mail through POP3 / IMAP

　　Your friend's client will regularly connect to their mail server; use POP3 to download mail from the server to the local computer, and can delete mail on the server; use IMAP mail to remain on the server, and the client only views it synchronously (suitable for multiple devices to access); this step is the process of "receiving" the mail.

　　Why is it not so easy to set up a mail server yourself?

　　Although it sounds logical, in reality SMTP is very easy to be abused to send spam; you need to configure SPF, DKIM, DMARC and other anti-counterfeiting verifications; set up anti-spam and anti-virus gateways; the IP needs to be "clean", otherwise it is easy to be blacklisted and emails cannot be sent; major mailboxes have very strict requirements for unfamiliar domain names. Therefore, many small and medium-sized enterprises and individual webmasters will choose to use third-party mail services to save the trouble of setting up their own post offices. But if you are familiar with servers and want to fully control the mail system, setting up your own Mail Server is also a very free and interesting choice.

　　Mail Server is the cornerstone of Internet communication. It is not as "visible" as the software on your computer, but it works silently every day; sending, receiving, verifying, storing, every step must strictly abide by the rules; for enterprises, a reliable mail server is the lifeblood of communication; for individuals, understanding its principles can better ensure data security and reduce spam.