为什么电子邮件客户端需要 SMTP 中继服务器?那有必要吗? [英] Why email client need a SMTP relay server? Is that neccessary?

问题描述

在设置我的移动邮件客户端时，我通常需要输入 SMTP 中继服务器的域名，但是为什么我的邮件客户端需要一个 SMTP 中继服务器呢?

When setting-up my mobile email client, I usually need to enter the SMTP relay server domain name, but why my email client need a SMTP relay server at all?

我认为它可以通过查找 DNS MX 记录，直接与目标电子邮件地址所在域的 smtpd 服务器(例如 postfix)对话.直接发送给它，何乐而不为呢?

I think it can directly talk to the smtpd server (e.g. postfix) from which domain the destination Email address is, by looking up the DNS MX records. And send directly to it, why not?

其实我还有一个困惑，如果一个 smtpd 服务器使用 SMTP AUTH，那么其他地方的依赖服务器如何依赖邮件给它呢?依赖服务器根本没有传输该邮件所需的用户名和密码.

Actually I have another confusion, if a smtpd server use SMTP AUTH, then how can a rely server elsewhere rely a mail to it? The rely server simply don't have the username and password necessary to transfer that mail.

推荐答案

邮件客户端使用 SMTP 中继服务器来

Email clients use SMTP relay server to

• 简化电子邮件发送(例如，避免电子邮件客户端重复发送尝试)
• 避免被误认为垃圾邮件发送者.

在古代垃圾邮件过去之前，直接发送电子邮件可能是一个相当简单的选择.

Direct email sending might be a reasonably simple option in ancient pre spam past.

SMTP AUTH(主要)用于电子邮件客户端与电子邮件服务器的通信.如果不禁止来自陌生人"的电子邮件，SMTP 服务器到 SMTP 服务器的通信就不能要求 SMTP AUTH.SMTP AUTH 允许接受来自受信任的客户端(客户)的消息，即使来自 IP 地址的消息也会被 DNSBL(例如 DUL 范围).

SMTP AUTH is used (mainly) with email client to email server communication. SMTP server to SMTP server communication could not require SMTP AUTH without prohibiting email from "strangers". SMTP AUTH allows to accept messages from trusted clients (customers) even from IP addresses that otherwise would be blocked by DNSBL (e.g. DUL ranges).

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