An email passes through multiple servers that help ensure it arrives 在 right place. That email message flow uses a systematic process based on a number of long-established technical standards.
准备好一睹Bird 的风采了吗？
了解电子邮件的流程，从发送到交付
For most users, how an email message flows from the sender to a recipient’s inbox is something that happens behind the scenes. When an individual or an organization sends an email, the message travels from its point of origination, such as an email client where it was composed, across the Internet to its destination. Along the way, it passes through multiple servers that help ensure it arrives at the right place. That email message flow uses a systematic process based on a number of long-established technical standards.
一封电子邮件的流程是怎样的？
Email has been around since the 1960s, when the creators of nascent computer networks began devising ways to send messages to each other. In those early days, users were limited to communicating only with others on the same shared mainframe system. However, the adoption of standard protocols and the interconnection of systems into the shared network we now know as the Internet allowed different mail systems to “talk” to each other.
It’s these standards that allow us to send email messages to virtually anyone. When someone sends an email message, it flows through a series of steps to reach its destination.
当一个人写信息时，通常是在Outlook或Apple Mail等电子邮件客户端中完成的，或者是在Gmail等基于网络的服务中。
However, when the message is a 事务性电子邮件 like a shipping notice or a password reset, the message is created automatically by those systems, usually using an email API. (Marketing messages are generated by automated systems as well, although usually in large batches, rather than one at a time like transactional messages.)
In both cases, whether the message is created by an email client or by an automated system, it is specially formatted to be transmitted over the Internet using a standard called “Simple Mail Transfer Protocol” (SMTP).
ǞǞǞ sender’s mail server (technically called a “Mail Transfer Agent,” or MTA) looks up the “@domain.com” portion of the recipient’s email address in a Domain Name System (DNS) server to determine which destination mail server (referred to as a “Mail Exchanger,” or MX) it should contact to deliver the message.
ǞǞǞ sending and receiving servers communicate using the SMTP protocol. The receiving server accepts the message so that it can be delivered to the recipient.
The recipient’s email client retrieves the message using standards like the Post Office Protocol (POP) or Internet Message Access Protocol (IMAP) to download the message so it can be read.
电子邮件客户端如何下载邮件？
Web-based email services like Gmail or Hotmail/Outlook.com use their own internal protocols to manage email. But when recipients use a stand-alone email client on a phone or desktop computer, that software uses standard protocols to download messages from mail servers.
当收件人使用POP时，服务器会将所有新的电子邮件传递给他们，只有当电子邮件客户端中的选项被选中时，才会保留它们的副本，如果适用。如果服务器没有电子邮件的副本，而收件人遭受硬件损失或故障，这些邮件就永远消失了，除非发件人有它们的副本。
当收件人使用IMAP时，服务器会将邮箱的内容，包括它的发件箱和其他文件夹，同步到每个与它连接的设备上。邮件保留在服务器上，当一个邮件的状态发生变化时（例如，它被读取或删除），当它们再次连接时，这种变化会在所有设备上传播。
在多个设备上保留和同步信息的能力是今天大多数电子邮件服务使用IMAP而不是POP的原因。
阅读更多关于电子邮件信息流
Email involves many different steps and systems. You can learn more about the inner workings of email delivery with these resources:
Email: This Wikipedia article delves into the history of email as well as its many technical details. It links out to several supporting articles that flesh out the main subject.
Email API: Learn more about how e-commerce and other systems can generate transactional email very efficiently.
云电子邮件传送: Learn more about how systems the SparkPost email delivery service work to make large-scale email transmission and delivery practical.