smtp自动发送邮件服务器_为什么需要中间SMTP服务器发送邮件？

smtp自动发送邮件服务器

As a person learns more about how mail clients, SMTP servers, and the whole online mail system works, they may be curious as to why an intermediate SMTP server is even needed. With that in mind, today’s SuperUser Q&A post has the answers to a curious reader’s questions.

当人们进一步了解邮件客户端，SMTP服务器以及整个联机邮件系统的工作方式时，他们可能会对为什么甚至需要中间SMTP服务器感到好奇。 考虑到这一点，今天的SuperUser Q＆A帖子回答了好奇的读者的问题。

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

今天的“问答”环节由SuperUser提供，它是Stack Exchange的一个分支，该社区是由社区驱动的Q＆A网站分组。

Photo courtesy of David Schroeder (Flickr).

照片由David Schroeder(Flickr)提供 。

问题 (The Question)

SuperUser reader Tobia wants to know why an intermediate SMTP server is needed to send mail:

超级用户阅读器Tobia想知道为什么需要一个中间SMTP服务器来发送邮件：

Why do I need an intermediate SMTP server to send mail? Why is my mail client (Outlook or Thunderbird) unable to send messages directly to the recipient’s SMTP domain?

为什么需要中间SMTP服务器来发送邮件？ 为什么我的邮件客户端(Outlook或Thunderbird)无法直接将邮件发送到收件人的SMTP域？

For example, if I have to send mail to [email protected] with my Gmail account, I send it to the smtp.gmail.com server; then this server sends my message to the MX server of example.com.

例如，如果我必须使用我的Gmail帐户将邮件发送到[email protected] ， 则将其发送到smtp.gmail.com服务器； 然后，该服务器将我的消息发送到example.com的MX服务器。

Why is an intermediate SMTP server needed to send mail?

为什么需要中间SMTP服务器发送邮件？

答案 (The Answer)

SuperUser contributor davidgo has the answer for us:

超级用户贡献者davidgo为我们提供了答案：

It is technically possible to send mail directly to the recipient’s SMTP server from your computer.

从技术上讲，可以从计算机直接将邮件发送到收件人的SMTP服务器。

Looking at it from a historical basis, if the remote SMTP server is down, you want a system to automatically handle it and keep retrying, hence you have an SMTP server. Similarly, in the old days, not all mail servers were connected all the time (long distance links were expensive), so mail would be queued and sent when a link was established.

从历史的角度来看，如果远程SMTP服务器已关闭，则您希望系统自动处理它并继续重试，因此您拥有SMTP服务器。 同样，在过去，并非所有邮件服务器都始终保持连接状态(长距离链接非常昂贵)，因此，在建立链接时，邮件将排队并发送。

Moving on to where Internet services are cheap, it is still useful to have mechanisms to retry sending mail if a server is unavailable. It is not ideal for this functionality to be written into the MUA (Mail user agent/end user mail program). These functions fit into an MTA (Mail server/SMTP server).

转移到Internet服务便宜的地方，如果服务器不可用，具有重试发送邮件的机制仍然很有用。 将此功能写入MUA(邮件用户代理/最终用户邮件程序)并不理想。 这些功能适合MTA(邮件服务器/ SMTP服务器)。

But it gets worse—spammers. Most mail (more than 80 percent) is spam. Mail providers do whatever they can to reduce this problem and a large number of techniques make assumptions about the way mail is delivered. The following are important considerations:

但情况变得更糟-垃圾邮件发送者。 大多数邮件(超过80％)是垃圾邮件。 邮件提供商竭尽所能减少此问题，并且大量技术都对邮件的传递方式进行了假设。 以下是重要的注意事项：

1. Greylisting: Some providers will automatically drop a mail connection if the sender and recipient have not communicated before and expect them to try a second time. Spammers often do not retry while an SMTP server is always supposed to. This reduces the volume of spam by about 80 percent, but it sucks to have to do this though.

1. 列入名单：如果发件人和收件人之前从未进行过通信，则某些提供商将自动断开邮件连接，并希望他们再次尝试。 垃圾邮件发送者通常不尝试重试，而总是应该使用SMTP服务器。 这样可以将垃圾邮件的数量减少约80％，但是必须这样做却很糟糕。

2. Reputation: It is a lot more likely that someone sending mail through a reputable, known SMTP server is legit compared to a fly-by-night server. To get a feel for reputation, providers do a number of things:

2. 信誉：与夜间飞行服务器相比，通过信誉良好的已知SMTP服务器发送邮件的人更有可能合法。 为了获得声誉，供应商做了很多事情：

• Block dynamic/client addresses (not 100 percent, but large chunks of the Internet have been mapped out).
  阻止动态/客户端地址(不是100％，但是已经映射了Internet的很大一部分)。
• Check to see if the reverse DNS matches the forward DNS. Not very hard to do, but it shows some level of accountability and knowledge of best practices (something a lot of client address blocks do not have).
  检查反向DNS是否与正向DNS相匹配。 并不是很困难，但是它显示了一定程度的责任心和最佳实践知识(很多客户端地址块没有)。
• Check for reputation. When communicating with other SMTP servers, a lot of providers keep track of the amount of spam and volume of mail sent. They can reduce the amount of spam by limiting connections and keeping an eye on these parameters. There are a lot of ways this is done, not all of them obvious, but which require a known sender.
  检查声誉。 与其他SMTP服务器通信时，许多提供商都跟踪垃圾邮件的数量和发送的邮件量。 通过限制连接并关注这些参数，它们可以减少垃圾邮件的数量。 有很多方法可以完成此操作，但并非所有方法都是显而易见的，但是需要已知的发送者。
• SPF and DKIM. These mechanisms tie DNS resources to the domain name to make forging mail harder and would be difficult, but not necessarily impossible to deploy if the mail program (MUA) is responsible for outgoing mail.
  SPF和DKIM。 这些机制将DNS资源与域名绑定在一起，从而使伪造邮件更加困难，并且将很困难，但如果邮件程序(MUA)负责传出邮件，则不一定会无法部署。

There are probably other minor concerns, but these would be the major ones.

可能还有其他一些小问题，但这些可能是主要问题。

Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

有什么补充说明吗？ 在评论中听起来不错。 是否想从其他精通Stack Exchange的用户那里获得更多答案？ 在此处查看完整的讨论线程 。

翻译自: https://www.howtogeek.com/235161/why-is-an-intermediate-smtp-server-needed-to-send-mail/

smtp自动发送邮件服务器