电子邮件经过多个服务器，这些服务器帮助确保它到达正确的位置。该电子邮件消息流使用基于若干长期建立的技术标准的系统化流程。
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理解电子邮件消息流，从发送到投递
对于大多数用户来说，电子邮件消息从发送者流向收件人的收件箱，这一过程大多是在幕后进行的。当个人或组织发送电子邮件时，消息从其起始点（例如，撰写电子邮件的电子邮件客户端）通过互联网传输到其目的地。在此过程中，它经过多个服务器，这些服务器有助于确保邮件能够到达正确的地方。这一电子邮件消息流采用基于多项长期建立的技术标准的系统化流程。
电子邮件是如何流动的？
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.
When an individual writes a message, it’s usually done in an email client like Outlook or Apple Mail—or in a web-based service like Gmail.
However, when the message is a transactional email 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).
The 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.
The 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.
When the recipient uses POP, the server delivers all new emails to them and only keeps copies of them if an option in the email client is checked, if applicable. If the server doesn’t have copies of the emails and the recipient suffers a hardware loss or failure, those messages are gone forever, unless the senders have copies of them.
When the recipient uses IMAP, the server syncs the contents of the mailbox, including its Sent Items and other folders, to each device that connects with it. The messages remain on the server, and when the status of one changes (for example, it’s read or deleted), that change propagates across all devices when they connect again.
The ability to retain and sync messages on multiple devices is why most email services today use IMAP instead of POP.
了解更多关于电子邮件消息流的信息
电子邮件涉及许多不同的步骤和系统。 您可以通过以下资源了解更多关于电子邮件投递的内在工作原理：
Email: 这篇维基百科文章深入探讨了电子邮件的历史以及其许多技术细节。它链接到几个支持文章，以充实主要主题。
Email API: 了解更多关于电子商务和其他系统如何非常高效地生成事务性电子邮件的信息。
Cloud Email Delivery: 了解更多关于系统如何通过SparkPost电子邮件投递服务工作以使大规模电子邮件传输和投递变得实用的信息。
