SMTPS

Key takeaways

SMTPS belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SMTPS to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SMTPS from memory before moving on to harder problems.

Reference excerpt

SMTPS (Simple Mail Transfer Protocol Secure) is a method for securing the SMTP using transport layer security. It is intended to provide authentication of the communication partners, as well as data integrity and confidentiality. SMTPS is neither a proprietary protocol nor an extension of SMTP. It is a way to secure SMTP at the transport layer, by wrapping SMTP inside Transport Layer Security (TLS). Conceptually, it is similar to how HTTPS wraps HTTP inside TLS. This means that the client and server speak normal SMTP at the application layer, but the connection is secured by SSL or TLS. This happens when the TCP connection is established, before any mail data has been exchanged. Since whether or not to use SSL or TLS is not explicitly negotiated by the peers, services that speak SMTPS are usually reachable on a dedicated port of their own.

Difference between SMTPS and smtps "smtps" is also the name of an IANA-registered service, with the TCP port number 465. The service was intended for use by Mail Transfer Agents (MTAs), as a point of contact where these could exchange email in an encrypted form rather than in plaintext. The registration was quickly revoked, however, as standardization efforts resulted in an alternate approach. The registration has never been reinstated. When describing the IANA service registration, the official capitalization is "smtps". When describing the network protocol, the capitalization "SMTPS" is often used (similar to how HTTPS is capitalized). Port 587 is the well-known port for submitting mail to a server, frequently (but not required to be) encrypted using STARTTLS. Some email service providers allow their customers to use the SMTPS protocol to access a TLS-encrypted version of the "submission" service on port 465. This is a different service from what the original IANA registration dedicated the port to (for it used to be dedicated to encrypted content delivered as-is / in plain text, whereas nowadays' SMTPS on port 465 still uses plaintext content, only wrapped within TLS-encrypted transportation-basically the reverse mechanism). RFC 8314 corrected this problem (a special exception was made to assign TCP-465 to two use cases simultaneously) and integrated the use of port 465 as a TLS-encrypted "submission" port into the well-known port registrations published by IANA. The service is named as submissions. Although the IANA registration for port 465 was revoked in 1998 when STARTTLS was standardised, leading to port 587 with STARTTLS becoming the de facto standard for submission, RFC 8314 reversed this position in 2018. The RFC explicitly recommends that connections to mail submission servers and mail access servers be made using "Implicit TLS", in preference to connecting to the port and negotiating TLS using STARTTLS. While there is no longer any officially registered endpoint for the SMTP service, it is still possible to exchange email over an encrypted transport with similar guarantees as those offered by smtps, in particular with the guarantee that either the exchange succeeds securely, or does not happen at all, by using DANE in combination with DNSSEC. Many email servers are configured to either not deliver email securely at all, or to first try secure delivery with the STARTTLS mechanism. If that fails, for example, because the remote service does not offer it, or because a successful MITM-attack has stripped announcement of the feature, it simply falls back to delivery by insecure means.

History In early 1997, the Internet Assigned Numbers Authority registered port 465 for smtps. Late 1998 this was revoked when STARTTLS was standardized. With STARTTLS, the same port can be used with or without TLS. The use of well-known ports for mail exchanges communicating with SMTP was discussed in particular at the time. Port 465 currently shows as registered for both Source-Specific Multicast and submissions. RFC 8314 "Cleartext Considered Obsolete: Use of TLS for Email Submission and Access" proposed to recognize port 465 for implicitly encrypted email submission officially. And the IANA registration was updated 2017-12-12 using a special IESG approval accordingly.

Worked examples

Example 1 — a first encounter with SMTPS

Start with the simplest possible case. Write down what SMTPS claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SMTPS before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SMTPS ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SMTPS

In research: SMTPS appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SMTPS in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SMTPS is common in secondary-school and first-year university syllabi. It links to neighbouring topics Internet mail protocols, Transport Layer Security, so understanding it makes those chapters shorter.
In everyday life: Look for SMTPS outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study SMTPS in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SMTPS means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SMTPS out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SMTPS in simple terms?

SMTPS (Simple Mail Transfer Protocol Secure) is a method for securing the SMTP using transport layer security. It is intended to provide authentication of the communication partners, as well as data integrity and confidentiality.

Why does SMTPS matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SMTPS?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SMTPS.