add chapter 15.12 (#689)

eBook/15.12.md

@@ -0,0 +1,82 @@
+# 15.12 用 smtp 发送邮件
+
+`smtp` 包实现了用于发送邮件的“简单邮件传输协议”（Simple Mail Transfer Protocol）。它有一个 `Client` 类型，代表一个连接到 SMTP 服务器的客户端：
+
+- `Dial` 方法返回一个已连接到 SMTP 服务器的客户端 `Client`
+- 设置 `Mail`（from，即发件人）和 `Rcpt`（to，即收件人）
+- `Data` 方法返回一个用于写入数据的 `Writer`，这里利用 `buf.WriteTo(wc)` 写入
+
+示例代码 15.26 [smtp.go](examples/chapter_15/smtp.go)
+```go
+package main
+
+import (
+	"bytes"
+	"log"
+	"net/smtp"
+)
+
+func main() {
+	// Connect to the remote SMTP server.
+	client, err := smtp.Dial("mail.example.com:25")
+	if err != nil {
+		log.Fatal(err)
+	}
+	// Set the sender and recipient.
+	client.Mail("sender@example.org")
+	client.Rcpt("recipient@example.net")
+	// Send the email body.
+	wc, err := client.Data()
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer wc.Close()
+	buf := bytes.NewBufferString("This is the email body.")
+	if _, err = buf.WriteTo(wc); err != nil {
+		log.Fatal(err)
+	}
+}
+```
+
+如果需要认证，或有多个收件人时，也可以用 `SendMail` 函数发送。它连接到地址为 `addr` 的服务器；如果可以，切换到 TLS（“传输层安全”加密和认证协议），并用 PLAIN 机制认证；然后以 `from` 作为发件人，`to` 作为收件人列表，`msg` 作为邮件内容，发出一封邮件：
+```go
+func SendMail(addr string, a Auth, from string, to []string, msg []byte) error
+```
+
+示例 15.27 [smtp_auth.go](examples/chapter_15/smtp_auth.go)
+```go
+package main
+
+import (
+	"log"
+	"net/smtp"
+)
+
+func main() {
+	// Set up authentication information.
+	auth := smtp.PlainAuth(
+		"",
+		"user@example.com",
+		"password",
+		"mail.example.com",
+	)
+	// Connect to the server, authenticate, set the sender and recipient,
+	// and send the email all in one step.
+	err := smtp.SendMail(
+		"mail.example.com:25",
+		auth,
+		"sender@example.org",
+		[]string{"recipient@example.net"},
+		[]byte("This is the email body."),
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+```
+
+## 链接
+
+- [目录](directory.md)
+- 上一节：[与 websocket 通信](15.11.md)
+- 下一节：[常见的陷阱与错误](16.0.md)

eBook/examples/chapter_15/smtp.go

@@ -0,0 +1,30 @@
+// smtp.go
+package main
+
+import (
+        "bytes"
+        "log"
+        "net/smtp"
+)
+
+func main() {
+        // Connect to the remote SMTP server.
+        client, err := smtp.Dial("mail.example.com:25")
+        if err != nil {
+                log.Fatal(err)
+        }
+        // Set the sender and recipient.
+        client.Mail("sender@example.org")
+        client.Rcpt("recipient@example.net")
+        // Send the email body.
+        wc, err := client.Data()
+        if err != nil {
+                log.Fatal(err)
+        }
+        defer wc.Close()
+        buf := bytes.NewBufferString("This is the email body.")
+        if _, err = buf.WriteTo(wc); err != nil {
+                log.Fatal(err)
+        }
+}
+

eBook/examples/chapter_15/smtp_auth.go

@@ -0,0 +1,29 @@
+// smtp_auth.go
+package main
+
+import (
+        "log"
+        "net/smtp"
+)
+
+func main() {
+        // Set up authentication information.
+        auth := smtp.PlainAuth(
+                "",
+                "user@example.com",
+                "password",
+                "mail.example.com",
+        )
+        // Connect to the server, authenticate, set the sender and recipient,
+        // and send the email all in one step.
+        err := smtp.SendMail(
+                "mail.example.com:25",
+                auth,
+                "sender@example.org",
+                []string{"recipient@example.net"},
+                []byte("This is the email body."),
+        )
+        if err != nil {
+                log.Fatal(err)
+        }
+}