服务器在来自电子邮件的图像请求中收到什么？
以这种情况为例：
- 我发送一封嵌入了图像的 HTML 电子邮件 - 例如使用标签将图像放置在新闻通讯中。
- 我打开电子邮件
- 请求图像
在该图像请求期间，发送到服务器的全套信息是什么？有人可以粘贴进去吗？
希望看到完整的请求和信息在 Outlook 等客户端和 Gmail 等至少一个网络邮件平台中打开电子邮件时的标头。
至于关于哪个服务器的评论——我猜是HTTP？如果我正在 Gmail 中查看电子邮件，并且提供了一个标签，那么该标签是通过 HTTP 提供的，对吧？
谢谢！
Take this scenario:
- I send an HTML email with images embedded in it - such as a newsletter with images placed inside using an tag.
- I open the email
- An image is requested
During that image request, what is the full set of information sent to the server? Can someone paste it in?
Would love to see the full request & header when the email is opened in a client like Outlook and at least one webmail platform like Gmail.
As to the comment about which server -- I guess HTTP? If I'm viewing an email in Gmail, and an tag is served, that is being served over HTTP, right?
Thanks!
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就客户端计算机而言，邮件客户端应用程序与基于 Web 的邮件前端完全不同。
通常，邮件客户端应用程序（正式的邮件用户代理或 MUA）使用特定于邮件的协议例如 POP3 或 IMAP 访问远程服务器上的邮箱。
根据协议的功能，MUA 可能必须在显示任何内容 (POP3) 之前下载整个消息，或者它可能能够请求每个 MIME 单独附件 (IMAP)。通常图像作为 MIME 附件包含并以可打印的形式编码（即限制为 7 位 ASCII) 使用 Base64 之类的东西，以确保交付不会损坏。
另一方面，基于 Web 的邮件前端是通过通用 HTTP 协议访问的 -消息解码发生在远程服务器上。与其他图像一样，图像是通过正常的 GET 请求检索的。
几年前，您可能能够使用网络嗅探器工具（例如 Wireshark）在工作中看到这两种情况。 。如今，大多数体面的邮件提供商更喜欢这些协议的加密版本（POP3S、IMAPS 和 HTTPS）无法使用网络嗅探器进行有用的研究。
如果您确实希望查看协议，您仍然可以找到允许未加密连接的 POP3/IMAP 服务，尽管它可能不会成为一个大的提供商。您还可以设置您自己的服务器进行实验。
另一方面，如果您使用Firefox，则至少有一个插件，允许您查看浏览器，通过在加密之前拦截请求。它可以让您看到使用 Gmail 时发生的情况。
As far as the client computer is concerned, mail client applications are completely different from a web-based mail front-end.
Typically, mail client applications (formally Mail User Agents or MUAs) use a mail-specific protocol such as POP3 or IMAP to access a mailbox on a remote server.
Depending on the capabilities of the protocol, the MUA may have to download a whole message before displaying anything (POP3) or it may be able to request each MIME attachment separately (IMAP). Usually images are included as MIME attachments and are encoded in a printable form (i.e. restricted to 7-bit ASCII) using something like Base64, in order to ensure delivery without corruption.
Web-based mail front-ends, on the other hand, are accessed through the generic HTTP protocol - the message decoding happens at the remote server. Images are retrieved through normal GET requests like every other image.
A few years back, you might have been able to see both cases at work using a network sniffer tool such as Wireshark. These days most decent mail providers prefer the encrypted versions of these protocols (POP3S, IMAPS and HTTPS) which cannot be usefully studied using a network sniffer.
If you really wish to have a look at the protocols, you might still be able to find a POP3/IMAP service that allows unencrypted connections, although it's probably not going to be a big provider. You could also set-up your own server for experimentation.
If you use Firefox, on the other hand, there is at least one add-on that allows you to see the requests made by the browser, by intercepting the requests before they are encrypted. It will allow you to see what happens as you use e.g. Gmail.