Simple Mail Transfer Protocol (SMTP)

Port

Port	Description
25	SMTP，最古老的，主要用來 Server ->> Server 之間的信件傳送
465	SMTPS，已棄用，建議使用 587，但實務上還是會使用
587	Submission，主要用來接收 SMTP Client 發送的信件
2525	Fallback，沒有在規範內

SMTP Server Minimum Implementation

根據 RFC 5321 的描述，這些是 SMTP Server 至少要有的功能

In order to make SMTP workable, the following minimum implementation
MUST be provided by all receivers.  The following commands MUST be
supported to conform to this specification:

  EHLO
  HELO
  MAIL
  RCPT
  DATA
  RSET
  NOOP
  QUIT
  VRFY

netcat 安裝

安裝 netcat 當作 SMTP Client 端，方便後續連線到 SMTP Server 輸入指令

brew install netcat

P.S. 之前有使用過 telnet，但終端介面的鍵盤事件很不習慣，例如 backspace 不能刪除打錯的字，以及 Ctrl + C 不能退出...

SMTP Commands

使用 https://ethereal.email/ 現成的 SMTP Server 來測試

連線

nc smtp.ethereal.email 587
220 smtp.ethereal.email ESMTP Welcome to Ethereal MSA

Status Code	Description
220	Service ready

HELO

HELO
501 Error: Syntax: HELO hostname
HELO localhost
250 smtp.ethereal.email Nice to meet you, 59-xxx-y-zzz.hinet-ip.hinet.net

Command	Description
HELO `<domain>`	HELLO，用來讓 Server 知道 Client 是誰

Status Code	Description
501	Syntax error in parameters or arguments
250	Requested mail action okay, completed

EHLO

EHLO localhost
250-smtp.ethereal.email Nice to meet you, 59-xxx-y-zzz.hinet-ip.hinet.net
250-PIPELINING
250-8BITMIME
250-SMTPUTF8
250-AUTH LOGIN PLAIN
250 STARTTLS

Command	Description
EHLO	Extended Hello，用來確認 Server 支援哪些 Extensions

openssl s_client -connect smtp.ethereal.email:587 -starttls smtp
...建立 TLS 連線的訊息

# 第一次先故意輸入錯誤的帳密
AUTH LOGIN
334 VXNlcm5hbWU6 # Username: 的 base64 編碼
123
334 UGFzc3dvcmQ6 # Password: 的 base64 編碼
456
535 Authentication failed

# 第二次再用真實的帳密登入
AUTH LOGIN
334 VXNlcm5hbWU6
bGVvcG9sZG8udHVyY290dGUyNEBldGhlcmVhbC5lbWFpbA==
334 UGFzc3dvcmQ6
empubm5SMlE0d2tOVnBnbmYx
235 Authentication successful

Command	Description
AUTH LOGIN	使用 base64 username + password 來登入，跟 HTTP Basic Auth 同樣概念
STARTTLS	建立 TLS 連線，跟 FTP 的 AUTH TLS 是同樣概念

Status Code	Description
503	Bad sequence of commands
538	Encryption required for requested authentication mechanism
535	Authentication credentials invalid
334	334 `<base64 challenge>`
235	Authentication Succeeded

閒置太久

421 Timeout - closing connection

Status Code	Description
421	Service not available, closing transmission channel

斷開連線

QUIT
221 Bye

Command	Description
QUIT	斷開連線

Status Code	Description
221	Service closing transmission channel

保持活躍

NOOP
250 OK

Command	Description
NOOP	我還活著！別斷開連線

VRFY

看起來 smtp.ethereal.email 似乎沒有開啟 VRFY 的功能

VRFY leopoldo.turcotte24@ethereal.email
252 Try to send something. No promises though

Command	Description
VRFY	VERIFY a user

Status Code	Description
252	Cannot VRFY user, but will accept message and attempt delivery

EXPN

EXPN admin
500 Error: command not recognized

Command	Description
EXPN	EXPAND a mailing list

Status Code	Description
500	Syntax error, command unrecognized

關於 VRFY 跟 EXPN，由於都有列舉使用者郵件地址的資安風險，所以 RFC 5321 有專門提到解法：

If a site disables these commands for security reasons, the SMTP
  server MUST return a 252 response, rather than a code that could be
  confused with successful or unsuccessful verification.

寄送郵件

先執行 openssl 連線 + 登入，之後開始寄送郵件

MAIL FROM:<leopoldo.turcotte24@ethereal.email>
250 Accepted
RCPT TO:<test.account@ethereal.email>
RENEGOTIATING
40D7FDD02E7F0000:error:0A00010A:SSL routines:can_renegotiate:wrong ssl version:../ssl/ssl_lib.c:2323:

看起來是開頭大寫的 R 會被 openssl 視為 RENEGOTIATING，改成小寫的重試一次

MAIL FROM:<leopoldo.turcotte24@ethereal.email>
250 Accepted
rCPT TO:<test.account@ethereal.email>
250 Accepted
DATA
354 End data with <CR><LF>.<CR><LF>
Date: Fri, 25 Jul 2025 14:30:00 +0800
From: userx@leopoldo.turcotte24@ethereal.email

helloworld
.
250 Accepted [STATUS=new MSGID=aIIQr6fe0eT1BCHlaIOCxqqYuaj5Gk85AAAACVuuft7tdhLnFwSkehrezwA]
QUIT
DONE

這次有成功了，不過剛好我是用 Windows 系統，所以 Enter 鍵會被轉譯成 \r\n。

如果是 linux 系統，需要將第一行加上 -crlf 參數，就可以確保 Enter 鍵是 \r\n，而不是單純的 \n

openssl s_client -connect smtp.ethereal.email:587 -starttls smtp -crlf

Command	Description
MAIL FROM	寄件人
RCPT TO	收件人
DATA	開始傳送郵件內容

Status Code	Description
354	Start mail input; end with `<CRLF>.<CRLF>`

觸發待投遞的郵件

ETRN
500 Syntax Error

ETRN localname
501 Syntax Error in Parameters

ETRN uu.net
458 Unable to queue messages for node uu.net

ETRN sigurd.innosoft.com
250 OK, queuing for node sigurd.innosoft.com started

ETRN innosoft.com
250 OK, queuing for node innosoft.com started

Command	Description
ETRN	Extended Turn

Status Code	Description
458	Unable to queue messages for node xxx

SMTP Extensions

SMTP Server Extensions	Description
PIPELINING	支援一次傳送多條 Commands
8BITMIME	SMTP Message 支援 128 ~ 255 對應的字符
SMTPUTF8	支援 UTF8（Server 必須也要支援 8BITMIME）
DSN	Delivery Status Notifications

SMTP Message

上面的寄送郵件範例，有演示了基本的 SMTP Message 傳輸格式

Date: Fri, 25 Jul 2025 14:30:00 +0800
From: userx@leopoldo.turcotte24@ethereal.email

helloworld
.

其實用 HTTP 的概念來理解，就會發現其實不難，基本上有 87% 像，可參考我寫過的 anatomy-of-an-http-message 這篇文章。SMTP Message 可以分成三個區塊

SMTP Headers

SMTP Body
.

SMTP Message Headers

這邊把常用的 SMTP Headers 列出來參考：

SMTP Header	Description
* Return-Path	郵件寄出失敗時，要通知誰由終端 SMTP Server 在 SMTP Headers 的第一行加入 Return-Path: MAIL FROM `Return-Path: <leopoldo.turcotte24@ethereal.email>`
* From	郵件內容的作者 `From: Leopoldo Turcotte <leopoldo.turcotte24@ethereal.email>` ❗可設定多個作者，但實務上好像很少這樣做
Sender	寄件者 `Sender: Leopoldo Turcotte <leopoldo.turcotte24@ethereal.email>` ❗如果 From 跟 Sender 是同一個人，則 Sender 可以省略
To	收件者，可複數 `To: <messageA@example.com> <messageB@example.com>`
Cc	副本 (Carbon Copy)，可複數 `Cc: <messageA@example.com> <messageB@example.com>`
Bcc	密件副本 (Blind Carbon Copy)，可複數 `Bcc: <messageA@example.com> <messageB@example.com>`
Reply To	請回覆到這個郵件地址 `Reply To: "My Personal Account" <myPersonalAccount@ethereal.email>`
Subject	標題
Message-ID	`Message-ID: <c112c786-81a9-aa6c-ad3f-de01c887ae8a@ethereal.email>`
In Reply To	回覆哪一則郵件 `In Reply To: <c112c786-81a9-aa6c-ad3f-de01c887ae8a@ethereal.email>`
References	對話串的歷史 `References: <messageA@example.com> <messageB@example.com>`
Content-Transfer-Encoding	SMTP 最初只有支援 7-bit ASCII 文字為了要傳輸各國文字、二進制圖片等等，才有了這個 Header `Content-Transfer-Encoding: 7bit`
* Date	`Date: Sat, 26 Jul 2025 02:01:28 +0000`
MIME-Version	`MIME-Version: 1.0`
Content-Type	基本上跟 HTTP 的 Content-Type 是同樣概念

nodemailer

nodemailer 是一個 npm 知名的 SMTP Client，用來學習 SMTP 非常適合，並且它也有測試用的 SMTP Server 可以看到信件內容

我們先到 https://ethereal.email/ 免費註冊，並且把帳密跟使用者名稱記下來
npm i nodemailer
npm i -D @types/nodemailer
建立跟 SMTP Server 的連線

import { createTransport } from "nodemailer";

const transport = createTransport({
  debug: true,
  logger: true,
  host: "smtp.ethereal.email",
  port: 587,
  auth: {
    user: "leopoldo.turcotte24@ethereal.email",
    pass: "zjnnnR2Q4wkNVpgnf1",
  },
});

寄送

transport
  .sendMail({
    from: '"中文" 中文1@ethereal.email',
    to: ["中文2@ethereal.email", "中文3@ethereal.email"],
    subject: "中文 subject",
    text: "中文 content",
    html: "<h1>中文中文中文</h1>",
    cc: ["中文4@ethereal.email", "中文5@ethereal.email"],
    bcc: ["中文6@ethereal.email", "中文7@ethereal.email"],
    sender: "中文8@ethereal.email",
    replyTo: ["中文9@ethereal.email", "中文10@ethereal.email"],
    list: {
      Help: {
        url: "https://example.com/Help",
        comment: "Help",
      },
      Unsubscribe: {
        url: "https://example.com/Unsubscribe",
        comment: "Unsubscribe",
      },
      Subscribe: {
        url: "https://example.com/Subscribe",
        comment: "Subscribe",
      },
      Post: {
        url: "https://example.com/Post",
        comment: "Post",
      },
      Owner: {
        url: "https://example.com/Owner",
        comment: "Owner",
      },
      Archive: {
        url: "https://example.com/Archive",
        comment: "Archive",
      },
    },
    attachments: [
      { path: join(__dirname, "small.png"), filename: "small.png" },
    ],
  })
  .then((res) => console.log(res))
  .catch((err) => console.log(err));

DEBUG LOG

DEBUG [JEYEJIdafM] S: 220 smtp.ethereal.email ESMTP Welcome to Ethereal MSA
DEBUG [JEYEJIdafM] C: EHLO [127.0.0.1]
DEBUG [JEYEJIdafM] S: 250-smtp.ethereal.email Nice to meet you, 1-xxx-yyy-zz.dynamic-ip.hinet.net
DEBUG [JEYEJIdafM] S: 250-PIPELINING
DEBUG [JEYEJIdafM] S: 250-8BITMIME
DEBUG [JEYEJIdafM] S: 250-SMTPUTF8
DEBUG [JEYEJIdafM] S: 250-AUTH LOGIN PLAIN
DEBUG [JEYEJIdafM] S: 250 STARTTLS
DEBUG [JEYEJIdafM] C: STARTTLS
DEBUG [JEYEJIdafM] S: 220 Ready to start TLS
DEBUG [JEYEJIdafM] C: EHLO [127.0.0.1]
DEBUG [JEYEJIdafM] S: 250-smtp.ethereal.email Nice to meet you, 1-xxx-yyy-zz.dynamic-ip.hinet.net
DEBUG [JEYEJIdafM] S: 250-PIPELINING
DEBUG [JEYEJIdafM] S: 250-8BITMIME
DEBUG [JEYEJIdafM] S: 250-SMTPUTF8
DEBUG [JEYEJIdafM] S: 250 AUTH LOGIN PLAIN
DEBUG [JEYEJIdafM] SMTP handshake finished
DEBUG [JEYEJIdafM] C: AUTH PLAIN AGxlb3BvbGRvLnR1cmNvdHRlMjRAZXRoZXJlYWwuZW1haWwALyogc2VjcmV0ICov
DEBUG [JEYEJIdafM] S: 235 Authentication successful
DEBUG [JEYEJIdafM] C: MAIL FROM:<中文1@ethereal.email> SMTPUTF8
DEBUG [JEYEJIdafM] S: 250 Accepted
DEBUG [JEYEJIdafM] C: RCPT TO:<中文2@ethereal.email>
DEBUG [JEYEJIdafM] C: RCPT TO:<中文3@ethereal.email>
DEBUG [JEYEJIdafM] C: RCPT TO:<中文4@ethereal.email>
DEBUG [JEYEJIdafM] C: RCPT TO:<中文5@ethereal.email>
DEBUG [JEYEJIdafM] C: RCPT TO:<中文6@ethereal.email>
DEBUG [JEYEJIdafM] C: RCPT TO:<中文7@ethereal.email>
DEBUG [JEYEJIdafM] S: 250 Accepted
DEBUG [JEYEJIdafM] S: 250 Accepted
DEBUG [JEYEJIdafM] S: 250 Accepted
DEBUG [JEYEJIdafM] S: 250 Accepted
DEBUG [JEYEJIdafM] S: 250 Accepted
DEBUG [JEYEJIdafM] S: 250 Accepted
DEBUG [JEYEJIdafM] C: DATA
DEBUG [JEYEJIdafM] S: 354 End data with <CR><LF>.<CR><LF>
DEBUG [JEYEJIdafM] From: =?UTF-8?B?5Lit5paH?= <ä¸­æ1@ethereal.email>
DEBUG [JEYEJIdafM] Sender: ä¸­æ8@ethereal.email
DEBUG [JEYEJIdafM] To: ä¸­æ2@ethereal.email, ä¸­æ3@ethereal.email
DEBUG [JEYEJIdafM] Cc: ä¸­æ4@ethereal.email, ä¸­æ5@ethereal.email
DEBUG [JEYEJIdafM] Reply-To: ä¸­æ9@ethereal.email, ä¸­æ10@ethereal.email
DEBUG [JEYEJIdafM] Subject: =?UTF-8?Q?=E4=B8=AD=E6=96=87_subject?=
DEBUG [JEYEJIdafM] Message-ID: <bb6aa2ab-cfe6-67bf-9132-4e72509b4581@ethereal.email>
DEBUG [JEYEJIdafM] List-Help: <https://example.com/Help> (Help)
DEBUG [JEYEJIdafM] List-Unsubscribe: <https://example.com/Unsubscribe> (Unsubscribe)
DEBUG [JEYEJIdafM] List-Subscribe: <https://example.com/Subscribe> (Subscribe)
DEBUG [JEYEJIdafM] List-Post: <https://example.com/Post> (Post)
DEBUG [JEYEJIdafM] List-Owner: <https://example.com/Owner> (Owner)
DEBUG [JEYEJIdafM] List-Archive: <https://example.com/Archive> (Archive)
DEBUG [JEYEJIdafM] Date: Sun, 27 Jul 2025 02:58:50 +0000
DEBUG [JEYEJIdafM] MIME-Version: 1.0
DEBUG [JEYEJIdafM] Content-Type: multipart/mixed; boundary="--_NmP-bdcd17b03de1a474-Part_1"
DEBUG [JEYEJIdafM]
DEBUG [JEYEJIdafM] ----_NmP-bdcd17b03de1a474-Part_1
DEBUG [JEYEJIdafM] Content-Type: multipart/alternative;
DEBUG [JEYEJIdafM]  boundary="--_NmP-bdcd17b03de1a474-Part_2"
DEBUG [JEYEJIdafM]
DEBUG [JEYEJIdafM] ----_NmP-bdcd17b03de1a474-Part_2
DEBUG [JEYEJIdafM] Content-Type: text/plain; charset=utf-8
DEBUG [JEYEJIdafM] Content-Transfer-Encoding: quoted-printable
DEBUG [JEYEJIdafM]
DEBUG [JEYEJIdafM] =E4=B8=AD=E6=96=87 content
DEBUG [JEYEJIdafM] ----_NmP-bdcd17b03de1a474-Part_2
DEBUG [JEYEJIdafM] Content-Type: text/html; charset=utf-8
DEBUG [JEYEJIdafM] Content-Transfer-Encoding: base64
DEBUG [JEYEJIdafM]
DEBUG [JEYEJIdafM] PGgxPuS4reaWh+S4reaWh+S4reaWhzwvaDE+
DEBUG [JEYEJIdafM] ----_NmP-bdcd17b03de1a474-Part_2--
DEBUG [JEYEJIdafM]
DEBUG [JEYEJIdafM] ----_NmP-bdcd17b03de1a474-Part_1
DEBUG [JEYEJIdafM] Content-Type: image/png; name=small.png
DEBUG [JEYEJIdafM] Content-Transfer-Encoding: base64
DEBUG [JEYEJIdafM] Content-Disposition: attachment; filename=small.png
DEBUG [JEYEJIdafM]
DEBUG [JEYEJIdafM] iVBORw0KGgoAAAANSUhEUgAAAMgAAACqCAYAAADoZADPAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJ
...省略 base64 圖片的部分
DEBUG [JEYEJIdafM] ----_NmP-bdcd17b03de1a474-Part_1--
DEBUG [JEYEJIdafM] .
DEBUG [JEYEJIdafM] S: 250 Accepted [STATUS=new MSGID=aIIQr6fe0eT1BCHlaIWV7zr3pD0h7PfsAAAAG70xFV0ufO5Yy1T1yiO87Tw]
DEBUG [JEYEJIdafM] Closing connection to the server using "end"

sendMail response

{
  accepted: [
    '中文2@ethereal.email',
    '中文3@ethereal.email',
    '中文4@ethereal.email',
    '中文5@ethereal.email',
    '中文6@ethereal.email',
    '中文7@ethereal.email'
  ],
  rejected: [],
  ehlo: [ 'PIPELINING', '8BITMIME', 'SMTPUTF8', 'AUTH LOGIN PLAIN' ],
  envelopeTime: 1319,
  messageTime: 285,
  messageSize: 13395,
  response: '250 Accepted [STATUS=new MSGID=aIIQr6fe0eT1BCHlaIWV7zr3pD0h7PfsAAAAG70xFV0ufO5Yy1T1yiO87Tw]',
  envelope: {
    from: '中文1@ethereal.email',
    to: [
      '中文2@ethereal.email',
      '中文3@ethereal.email',
      '中文4@ethereal.email',
      '中文5@ethereal.email',
      '中文6@ethereal.email',
      '中文7@ethereal.email'
    ]
  },
  messageId: '<bb6aa2ab-cfe6-67bf-9132-4e72509b4581@ethereal.email>'
}

有幾個重點可以關注

STARTTLS 前後都會進行一次 EHLO
AUTH PLAIN 的格式，根據 RFC 4616

# 原始傳輸
AUTH PLAIN AGxlb3BvbGRvLnR1cmNvdHRlMjRAZXRoZXJlYWwuZW1haWwALyogc2VjcmV0ICov

# base64 decode
\x00leopoldo.turcotte24@ethereal.email\x00/* secret */

# 格式
UTF8NUL authcid UTF8NUL passwd

從 MAIL FROM:<中文1@ethereal.email> SMTPUTF8 可以發現 SMTPUTF8 真的有啟用
To, Cc, Bcc 這三個 SMTP Message Headers 實際上在 SMTP Command 是用多行 RCPT TO 來指定多個收件人

RCPT TO:<中文2@ethereal.email>
RCPT TO:<中文3@ethereal.email>
RCPT TO:<中文4@ethereal.email>
RCPT TO:<中文5@ethereal.email>
RCPT TO:<中文6@ethereal.email>
RCPT TO:<中文7@ethereal.email>

SMTP Message Headers 在傳輸 UTF-8 字串時，會經過 base64 encoding 再傳輸
（為了向後兼容，畢竟 SMTP 最初只支援 7-bit ASCII）

# 原始傳輸
From: =?UTF-8?B?5Lit5paH?= <ä¸­æ1@ethereal.email>

# 可以解讀成
From: =?UTF-8?Base64?中文?= <中文1@ethereal.email>

SMTP Message Body 若包含文字、HTML、圖片，就會啟動 Content-Type: multipart/mixed 傳輸
（跟 multipart/formdata 有異曲同工之妙）

text（quoted-printable 可參考 RFC 2045）

Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

=E4=B8=AD=E6=96=87 content

html

Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: base64

PGgxPuS4reaWh+S4reaWh+S4reaWhzwvaDE+

attachment

Content-Type: image/png; name=small.png
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=small.png

iVBORw0KGgoAAAANSUhEUgAAAMgAAACqCAYAAADoZADPAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJ
...省略 base64 圖片的部分

other status code

實務上在測試 Production 的 SMTP Server 時，我發現還有一些 status code 是前面沒有涵蓋到的

Status Code	Description
502	Command not implemented
450	Requested mail action not taken: mailbox unavailable
554	Transaction failed
535	Authentication credentials invalid
550	Requested action not taken: mailbox unavailable
454	Temporary authentication failure

enhanced status code

一開始在研究 SMTP 的時候，有看到這個概念，但由於會有一些資安疑慮，例如 CWE-203: Observable Discrepancy，所以就沒深挖。結果沒想到，實務上在測試 Production 的 SMTP Server 時，還真的有遇到開啟 enhanced status code 的服務，所以就趁這個機會來學習一下

class	Description
2.XXX.XXX	Success
4.XXX.XXX	Persistent Transient Failure
5.XXX.XXX	Permanent Failure

subject	Description
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

X.0.XXX Other or Undefined Status

Detail	Description
X.0.0	Other undefined Status

X.1.XXX Address Status

Detail	Description
X.1.0	Other address status
X.1.1	Bad destination mailbox address
X.1.2	Bad destination system address
X.1.3	Bad destination mailbox address syntax
X.1.4	Destination mailbox address ambiguous
X.1.5	Destination address valid
X.1.6	Destination mailbox has moved, No forwarding address
X.1.7	Bad sender's mailbox address syntax
X.1.8	Bad sender's system address

X.2.XXX Mailbox Status

Detail	Description
X.2.0	Other or undefined mailbox status
X.2.1	Mailbox disabled, not accepting messages
X.2.2	Mailbox full
X.2.3	Message length exceeds administrative limit
X.2.4	Mailing list expansion problem

X.3.XXX Mail System Status

Detail	Description
X.3.0	Other or undefined mail system status
X.3.1	Mail system full
X.3.2	System not accepting network messages
X.3.3	System not capable of selected features
X.3.4	Message too big for system
X.3.5	System incorrectly configured

X.4.XXX Network and Routing Status

Detail	Description
X.4.0	Other or undefined network or routing status
X.4.1	No answer from host
X.4.2	Bad connection
X.4.3	Directory server failure
X.4.4	Unable to route
X.4.5	Mail system congestion
X.4.6	Routing loop detected
X.4.7	Delivery time expired

X.5.XXX Mail Delivery Protocol Status

Detail	Description
X.5.0	Other or undefined protocol status
X.5.1	Invalid command
X.5.2	Syntax error
X.5.3	Too many recipients
X.5.4	Invalid command arguments
X.5.5	Wrong protocol version

X.6.XXX Message Content or Message Media Status

Detail	Description
X.6.0	Other or undefined media error
X.6.1	Media not supported
X.6.2	Conversion required and prohibited
X.6.3	Conversion required but not supported
X.6.4	Conversion with loss performed
X.6.5	Conversion Failed

X.7.XXX Security or Policy Status

Detail	Description
X.7.0	Other or undefined security status
X.7.1	Delivery not authorized, message refused
X.7.2	Mailing list expansion prohibited
X.7.3	Security conversion required but not possible
X.7.4	Security features not supported
X.7.5	Cryptographic failure
X.7.6	Cryptographic algorithm not supported
X.7.7	Message integrity failure

DSN

Notify

RCPT TO:<receiver@gmail.com> NOTIFY=NEVER
RCPT TO:<receiver@gmail.com> NOTIFY=SUCCESS,FAILURE,DELAY

ORCPT (Original Recipient)

RCPT TO:<receiver@gmail.com> NOTIFY=NEVER ORCPT=rfc822;test@gmail.com

RET

RET = Returned SMTP Message

MAIL FROM:<sender@gmail.com> RET=FULL
MAIL FROM:<sender@gmail.com> RET HDRS

ENVID

MAIL FROM:<sender@gmail.com> ENVID=newsletter-20250801-001 RET=HDRS
...
DATA
Original-Envelope-ID: newsletter-20250801-001

小結

SMTP 在 2025 年其實也是佔有一席之地，從它的 RFC 發展活躍度，我才發現到 SMTP 遠沒有我想像的簡單。這篇文章頂多算是淺入淺出的學習筆記，實際上還有很多主題我沒有研究，但我的目標只是對 SMTP 有個初步的認識，會用一些基本的 SMTP Commands，並且大概知道 SMTP Message 的格式。不過，有 HTTP 的基礎，再來學習 SMTP 真的會快很多，很多概念我認為都是相通的，在學習 SMTP 的時候，常常有一種忽然打通任督二脈的感覺，原來這不是 HTTP 獨有的設計模式啊！

參考資料

Topic	URL
Simple Mail Transfer Protocol	https://datatracker.ietf.org/doc/html/rfc5321
Internet Message Format	https://datatracker.ietf.org/doc/html/rfc5322
List-* 開頭的 SMTP Message Headers	https://datatracker.ietf.org/doc/html/rfc2369
PIPELINING	https://datatracker.ietf.org/doc/html/rfc2920
Authentication	https://datatracker.ietf.org/doc/html/rfc4954
8BITMIME	https://datatracker.ietf.org/doc/html/rfc6152
SMTPUTF8	https://datatracker.ietf.org/doc/html/rfc6531
Returning Enhanced Error Codes	https://datatracker.ietf.org/doc/html/rfc2034
Enhanced Mail System Status Codes	https://datatracker.ietf.org/doc/html/rfc3463
Delivery Status Notifications	https://datatracker.ietf.org/doc/html/rfc3461
Remote Message Queue Starting	https://datatracker.ietf.org/doc/html/rfc1985
NodeJS SMTP Client	https://github.com/nodemailer/nodemailer
nodemailer Official Document	https://nodemailer.com/
nodemailer Test SMTP Server	https://ethereal.email/

Port​

SMTP Server Minimum Implementation​

netcat 安裝​

SMTP Commands​

連線​

HELO​

EHLO​

登入​

閒置太久​

斷開連線​

保持活躍​

VRFY​

EXPN​

寄送郵件​

觸發待投遞的郵件​

SMTP Extensions​

SMTP Message​

SMTP Message Headers​

nodemailer​

other status code​

enhanced status code​

X.0.XXX Other or Undefined Status​

X.1.XXX Address 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 Message Media Status​

X.7.XXX Security or Policy Status​

DSN​

Notify​

ORCPT (Original Recipient)​

RET​

ENVID​

小結​

參考資料​