当一个人对邮件客户机、SMTP服务器以及整个在线邮件系统的工作原理有了更多的了解，他们可能会好奇为什么需要一个中间的SMTP服务器。有鉴于此，今天的超级用户问答帖子为好奇的读者提供了答案。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

图片由大卫施罗德（Flickr）提供。

问题

超级用户读者Tobia想知道为什么需要中间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?

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

为什么需要中间SMTP服务器来发送邮件？

答案

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.

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.

Moving on to where Internet services are cheap, it is still useful to have mechani**s 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 functi*** fit into an MTA (Mail server/SMTP server).

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 assumpti*** about the way mail is delivered. The following are important c***iderati***:

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.

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:

• Block dynamic/client addresses (not 100 percent, but large chunks of the Internet have been mapped out).
• 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).
• 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 connecti*** 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.
• SPF and DKIM. These mechani**s 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 resp***ible for outgoing mail.

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

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