能够用自己的域名发送电子邮件是非常好的，但是当有人开始用你的域名发送垃圾邮件时，你会怎么做呢？今天的超级用户问答文章讨论了这个问题，以帮助一个沮丧的读者。

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。

照片由Maria Elena（Flickr）提供。

问题

超级用户读者Jake M想知道某人是如何使用个人域发送电子邮件的：

Spammers or someone similar is sending e-mails to people using our domain name.

• The e-mails are from a user we did not create called: [email protected].
• The e-mail is to: [email protected].
• The content of the e-mail talks about a stock that is six cents but will go to fifteen cents and that people should buy it. It contains a link to Yahoo’s finance website, but I will not click it, so I am not sure if it is legitimate. We know of the e-mails because we get bounce-backs (the recipient must not exist).

What would allow someone (or a bot) to send an e-mail under our domain name? Is there anything we can do to stop this? Is this Dictionary Spamming?

有人是如何做到这一点的？有什么办法可以缓解这种情况吗？

答案

超级用户贡献者Paul和AFH为我们提供了答案。首先，保罗：

The SMTP protocol does not include any controls over the From and To fields in an e-mail. They can be whatever you like provided you have authority to send e-mails using the SMTP server.

So the short answer is nothing prevents anyone from using your domain in e-mails they send. Even normal users can put whatever e-mail address they like in their e-mail settings.

Spammers routinely use valid domain names as From addresses to avoid being blocked.

While you cannot stop someone from sending e-mails with your domain name, you can help e-mail servers around the world understand if e-mails sent from your domain name actually originated from you and are legitimate e-mails, so that any others can be discarded as spam.

SPF

One way is to use SPF. This is a record that goes into DNS and lets the Internet know what servers are permitted to send e-mails on behalf of your domain. It looks like this:

• ourdomain.com.au.  IN TXT “v=spf1 mx ip4:123.123.123.123 -all”

This says that the only valid sources of e-mail for ourdomain.com.au are the MX servers – the server defined as the recipient of e-mails for the domain, and another server at 123.123.123.123. E-mail from any other server should be c***idered spam.

Most e-mail servers will check for the presence of this DNS record and act accordingly.

DKIM

While SPF is easy to set up, DKIM takes a little more effort and should be implemented by your e-mail server administrator. If you send your e-mail via an ISP e-mail server, they will often have methods for quick setup of DKIM.

DKIM works similarly to SSL certificates. A public/private key pair is generated. The private key is known only to the e-mail server, and it will sign any outgoing e-mails.

The public key is published using DNS. So any server receiving e-mails marked as coming from your domain can check that the e-mail was signed by retrieving the public key and checking the signature in the e-mails. If no signature is present, or it is incorrect, the e-mail can be c***idered spam.

接下来是AFH的回答：

An e-mail can contain any Reply-To address you choose. Some e-mail servers will send undeliverable notificati*** back to the Reply-To address rather than the originator. Online mail handlers like Gmail require you to validate any Reply-To address you use when composing online, but there is no such restriction when using a remote client with POP3/IMAP. And if you run your own e-mail server, you can probably also fake the From address.

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