SMTP and POP3 server

SMTP and POP3 server
When you press the “Send” button in your e-mail program (e-mail client) the program will connect to a server on the network / Internet that is called an SMTP server. SMTP is an acronym for Simple Mail Transfer Protocol and it is a protocol that is used when e-mails are delivered from clients to servers and from servers to other servers.

When you download e-mails to your e-mail program the program will connect to a server on the net that is known as a POP3 server. A POP3 server uses a protocol named POP3 for its communication. That is the reason why it is called a POP3 server and POP3 is an acronym for Post Office Protocol version 3.

Post Office Protocol (POP) is het meestgebruikte protocol voor het ophalen van e-mail van een mailserver. POP3, de meest recente versie van POP, is een internetstandaard voor het overbrengen van e-mail van een server naar een client (e-mailprogramma van de gebruiker) over een TCP/IP-verbinding (gewoonlijk over poort 110). Bijna alle internetproviders bieden een e-mailaccount aan dat beschikbaar is via POP3.

Simple Mail Transfer Protocol-servers (SMTP) verwerken de verzending van uw e‑mailberichten naar internet. De SMTP-server verwerkt uitgaande e‑mail en wordt gebruikt in combinatie met een POP3 of IMAP inkomende e‑mailserver.

Role of SMTP Servers

When a mail is sent, the machine connects to your mail server that is a centralized computer, managing a specific email service. Simple Mail Transfer Protocol (SMTP) is the server that is responsible for sending messages. It transfers the emails from one server to another in several steps. Every mail transit will have 2 addresses- one for receiver’s and sender’s email addresses.

Simplifying the explanation, an SMTP is just a pick-and-drop server that picks sender emails and drops them into the destined recipients.

When is POP3 Used?

POP3 is largely linked with client programs like Thunderbird, Eudora, and Outlook. The email client screens out the server with provided login details. The POP3 scans the server looking for messages and downloads them to respective addresses. It removes your email soon from their server, once the messages get transferred to your account. In simpler words, POP3 will allow email for download and doesn’t keep a copy of the mail, once downloaded and delivered to client system.

Mail server macine has both POP3 service and SMTP service:

SMTP is responsible for moving the message from the sending computer to the recipient’s mail server.

Here’s a step-by-step (mail server software independent!) overview of how mail moves from sender to recipient:

John sends an e-mail to
John’s computer uses SMTP to transmit the message to his ISP’s server, ISP-SERVER
ISP-SERVER knows it isn’t “authoritative” for the e-mail domain, so it performs a DNS lookup for one that is (specifically ISP-SERVER looks for an MX record in the domain)
A DNS MX record reports that the server MAIL-SERVER accepts messages for the e-mail domain, so ISP-SERVER sends John’s message via the SMTP protocol to MAIL-SERVER
MAIL-SERVER has a mailbox for Dave, so it puts John’s e-mail in it
When Dave checks his e-mail, his computer uses the POP3 protocol to connect to MAIL-SERVER and download the message from his friend Dave.

Most often messages are delivered like this, from the sender’s mail server directly to the recipient’s. However, it’s perfectly possible that there could be multiple servers between the sender’s and the recipient’s, in which case each one acts as a “relay server” (reasons for this include having a server perform spam/virus filtering before sending the message to the destination server, or the destination company having many internal servers involved in moving the e-mail from one Internet-connected mail server to a server in the exact office where Dave works).

In any case, each relay server will look up a server that it should send the e-mail to (based on the domain name in the To: address of the e-mail) and use the SMTP protocol to pass the message on. Only once the message arrives at the destination server (where the recipient’s mailbox exists) will the POP3 protocol be used by the recipient’s computer to retrieve the message out of the user’s mailbox.

Good one:

POP3是Post Office Protocol 3的简称,即邮局协议的第3个版本,它规定怎样将个人计算机连接到Internet的邮件服务器和下载电子邮件的电子协议。它是因特网电子邮件的第一个离线协议标准,POP3允许用户从服务器上把邮件存储到本地主机(即自己的计算机)上,同时删除保存在邮件服务器.


SMTP 的全称是“Simple Mail Transfer Protocol”,即简单邮件传输协议。它是一组用于从源地址到目的地址传输邮件的规范,通过它来控制邮件的中转方式。SMTP 协议属于 TCP/IP 协议簇,它帮助每台计算机在发送或中转信件时找到下一个目的地。SMTP 服务器就是遵循 SMTP 协议的发送邮件服务器。

SMTP 认证,简单地说就是要求必须在提供了账户名和密码之后才可以登录 SMTP 服务器,这就使得那些垃圾邮件的散播者无可乘之机。
增加 SMTP 认证的目的是为了使用户避免受到垃圾邮件的侵扰。