How to create and send MIME messages with custom headers (e.g., X-headers) in Go?

To create and send MIME messages with custom headers in Go, you can use the "mime/multipart" and "net/smtp" packages. Below is an example code snippet:

package main

import (
	"bytes"
	"fmt"
	"net/smtp"
	"net/textproto"
)

func main() {
	// Set up custom headers
	headers := make(textproto.MIMEHeader)
	headers.Add("X-Custom-Header", "Value")

	// Set up multipart message
	multiPart := new(bytes.Buffer)
	writer := multipart.NewWriter(multiPart)
	part, err := writer.CreatePart(headers)
	if err != nil {
		fmt.Println("Failed to create MIME part:", err)
		return
	}
	part.Write([]byte("This is the message body"))

	// Close the multipart writer
	writer.Close()

	// Set up SMTP connection
	auth := smtp.PlainAuth("", "[email protected]", "your_password", "smtp.example.com")
	smtpAddr := "smtp.example.com:587"

	// Send the email with custom headers
	err = smtp.SendMail(smtpAddr, auth, "[email protected]", []string{"[email protected]"}, multiPart.Bytes())
	if err != nil {
		fmt.Println("Failed to send email:", err)
		return
	}

	fmt.Println("Email sent successfully")
}

In the above code, we first create a textproto.MIMEHeader to hold the custom headers. We add the custom header X-Custom-Header with value Value.

We then create a multipart.Writer and call its CreatePart method passing in the custom headers. We write the message body content to the created part.

After closing the writer, we set up the SMTP connection using smtp.PlainAuth with your email credentials and the SMTP server details. The SendMail function is then used to send the email, specifying the sender and recipient email addresses. The message content is passed as multiPart.Bytes().

If the email is sent successfully, it prints "Email sent successfully".