[网络领域顶会追踪系列#30][ATC'22]没那么简单：21世纪的email发送

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Not that Simple: Email Delivery in the 21st Century

作者单位：SBA Research，University of Vienna，TU Wien，Max-Planck-Institut für Informatik

背景

在过去二十年间，关于email及其安全性的RFC数量以及从小于100爆炸增长至接近500。这将SMTP协议（Simple Mail Transfer Protocol）嵌入到一个相互依存且与交付有关的标准的树中。

工作

在本文中，作者调查了真实世界的部署与这些日益复杂的交付和安全选择间的跟进情况。

为深入了解除生态中的巨头（Gmail、Outlook等）email发送的情况，作者让人们向11个不太的配置好的目标域发送email。

作者的测量允许他们评估email发送的核心方面，包括安全特性、DNS配置和在不同类型的提供商在IP版本支持方面的情况。

实验

作者发现新技术通常没有被充分支持，即使是在大型提供商那。

例如，在65.4%的email提供商能够通过IPv6解析主机的同时，只有44.3%可以通过IPv6发送邮件。

考虑安全特性，作者注意到少于一半（41.5%）的提供商依赖于DNSSEC验证解析器，并且加密大多是机会主义的（opportunistic），89.7%的提供商接受无效的证书。TLSA，作为一种基于DNS的证书验证方法，在作者的研究中仅仅被31.7%的提供商所使用。

最后，作者将注意力放在了现代标准对大量不请自来（unsolicited）的邮件（SPAM）的影响。作者发现灰名单是很有效的，在不影响正常发送的情况下，能够减少几乎一半的SPAM。然而，有趣的是，SPAM发送现在似乎聚焦于明文IPv4链接，让仅支持IPv6、强制TLS的入向email服务器为一个更加高效的反SPAM措施----尽管它也意味着拒绝大多数合法email。

原文摘要

Over the past two decades, the number of RFCs related to email and its security has exploded from below 100 to nearly 500. This embedded the Simple Mail Transfer Protocol (SMTP) into a tree of interdependent and delivery-relevant standards. In this paper, we investigate how far real-world deployments keep up with this increasing complexity of delivery- and security options. To gain an in-depth picture of email delivery apart from the giants in the ecosystem (Gmail, Outlook, etc.), we engage people to send emails to eleven differently configured target domains. Our measurements allow us to evaluate core aspects of email delivery, including security features, DNS configuration, and IP version support on the sending side across different types of providers.

We find that novel technologies are often insufficiently supported, even by large providers. For example, while 65.4\% of email providers can resolve hosts via IPv6, only 44.3\% can also deliver emails via IPv6. Concerning security features, we observe that less than half (41.5\%) of all providers rely on DNSSEC validating resolvers, and encryption is mostly opportunistic, with 89.7\% of providers accepting invalid certificates. TLSA, as a DNS-based certificate verification method, is only used by 31.7\% of the providers in our study. Finally, we turned our eye to the impact modern standards have on unsolicited bulk email (SPAM). We found that greylisting is effective, reducing the SPAM volume by roughly half while not impacting regular delivery. However, and interestingly, SPAM delivery currently seems to focus on plaintext IPv4 connections, making IPv6-only, TLS-enforcing inbound email servers a more effective anti-SPAM measure – even though it also means rejecting a major portion of legitimate emails.