science

List of SMTP server return codes

List of SMTP server return codes is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand List of SMTP server return codes rather than just read about it. In short: This is a list of Simple Mail Transfer Protocol (SMTP) response status codes. Status codes are issued by a server in response to a client's request made to the server.

Key takeaways

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

Reference excerpt

This is a list of Simple Mail Transfer Protocol (SMTP) response status codes. Status codes are issued by a server in response to a client's request made to the server. Unless otherwise stated, all status codes described here is part of the current SMTP standard, RFC 5321. The message phrases shown are typical, but any human-readable alternative may be provided.

Basic status code A "Basic Status Code" SMTP reply consists of a three digit number (transmitted as three numeric characters) followed by some text. The number is for use by automata (e.g., email clients) to determine what state to enter next; the text ("Text Part") is for the human user. The first digit denotes whether the response is good, bad, or incomplete:

2yz (Positive Completion Reply): The requested action has been successfully completed. 3yz (Positive Intermediate Reply): The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information. 4yz (Transient Negative Completion Reply): The command was not accepted, and the requested action did not occur. However, the error condition is temporary, and the action may be requested again. 5yz (Permanent Negative Completion Reply): The command was not accepted and the requested action did not occur. The SMTP client SHOULD NOT repeat the exact request (in the same sequence). The second digit encodes responses in specific categories:

x0z (Syntax): These replies refer to syntax errors, syntactically correct commands that do not fit any functional category, and unimplemented or superfluous commands. x1z (Information): These are replies to requests for information. x2z (Connections): These are replies referring to the transmission channel. x3z : Unspecified. x4z : Unspecified. x5z (Mail system): These replies indicate the status of the receiver mail system.

Enhanced status code The Basic Status Codes have been in SMTP from the beginning, with RFC 821 in 1982, but were extended rather extensively, and haphazardly so that by 2003 RFC 3463 rather grumpily noted that: "SMTP suffers some scars from history, most notably the unfortunate damage to the reply code extension mechanism by uncontrolled use." RFC 3463 defines a separate series of enhanced mail system status codes which is intended to be better structured, consisting of three numerical fields separated by ".", as follows:

class "." subject "." detail

class = "2" / "4" / "5"

subject = 1 to 3 digits

detail = 1 to 3 digits

The classes are defined as follows:

2.XXX.XXX Success: Report of a positive delivery action. 4.XXX.XXX Persistent Transient Failure: Message as sent is valid, but persistence of some temporary conditions has caused abandonment or delay. 5.XXX.XXX Permanent Failure: Not likely to be resolved by resending the message in current form. In general the class identifier MUST match the first digit of the Basic Status Code to which it applies. The subjects are defined as follows:

X.0.XXX Other or Undefined Status X.1.XXX Addressing Status X.2.XXX Mailbox Status X.3.XXX Mail System Status X.4.XXX Network and Routing Status X.5.XXX Mail Delivery Protocol Status X.6.XXX Message Content or Media Status X.7.XXX Security or Policy Status The meaning of the "detail" field depends on the class and the subject, and are listed in RFC 3463 and RFC 5248. A server capable of replying with an Enhanced Status Code MUST preface (prepend) the Text Part of SMTP Server responses with the Enhanced Status Code followed by one or more spaces. For example, the "221 Bye" reply (after QUIT command) MUST be sent as "221 2.0.0 Bye" instead. The Internet Assigned Numbers Authority (IANA) maintains the official registry of these enhanced status codes.

Common status codes This section list some of the more commonly encountered SMTP Status Codes. This list is not exhaustive, and the actual text message (outside of the 3-field Enhanced Status Code) might be different.

— 2yz Positive completion 211 System status, or system help reply 214 Help message (A response to the HELP command) 220 <domain> Service ready 221 <domain> Service closing transmission channel 221 2.0.0 Goodbye 235 2.7.0 Authentication succeeded 240 QUIT 250 Requested mail action okay, completed 251 User not local; will forward 252 Cannot verify the user, but it will try to deliver the message anyway

— 3yz Positive intermediate 334 (Server challenge - the text part contains the Base64-encoded challenge) 354 Start mail input

— 4yz Transient negative completion "Transient Negative" means the error condition is temporary, and the action may be requested again. The sender should return to the beginning of the command sequence (if any). The accurate meaning of "transient" needs to be agreed upon between the two different sites (receiver- and sender-SMTP agents) must agree on the interpretation. Each reply in this category might have a different time value, but the SMTP client SHOULD try again.

421 Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down) 432 4.7.12 A password transition is needed 450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons) 451 Requested action aborted: local error in processing 451 4.4.1 IMAP server unavailable 452 Requested action not taken: insufficient system storage 454 4.7.0 Temporary authentication failure 455 Server unable to accommodate parameters

— 5yz Permanent negative completion The SMTP client SHOULD NOT repeat the exact request (in the same sequence). Even some "permanent" error conditions can be corrected, so the human user may want to direct the SMTP client to reinitiate the command sequence by direct action at some point in the future.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with List of SMTP server return codes

Start with the simplest possible case. Write down what List of SMTP server return codes 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 List of SMTP server return codes 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 List of SMTP server return codes 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 List of SMTP server return codes

In research: List of SMTP server return codes 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 List of SMTP server return codes 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: List of SMTP server return codes is common in secondary-school and first-year university syllabi. It links to neighbouring topics Internet-related lists, so understanding it makes those chapters shorter.
In everyday life: Look for List of SMTP server return codes 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.

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How to study List of SMTP server return codes in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what List of SMTP server return codes 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 List of SMTP server return codes out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is List of SMTP server return codes in simple terms?

This is a list of Simple Mail Transfer Protocol (SMTP) response status codes. Status codes are issued by a server in response to a client's request made to the server.

Why does List of SMTP server return codes 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 List of SMTP server return codes?

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 List of SMTP server return codes.