How to parse and process email headers (e.g., From, To, Subject) from MIME messages in Go?

To parse and process email headers from MIME messages in Go, you can use the net/mail package. Here's an example of how you can accomplish this:

package main

import (
	"fmt"
	"net/mail"
)

func main() {
	// Example MIME message
	message := []byte(`Subject: Hello from Go!
To: John Doe <[email protected]>
From: Jane Smith <[email protected]>
Date: Tue, 01 Jan 2022 12:00:00 +0000
Content-Type: text/plain; charset="UTF-8"

This is the body of the email.`)

	// Parse the MIME message
	msg, err := mail.ReadMessage(bytes.NewReader(message))
	if err != nil {
		panic(err)
	}

	// Get the email headers
	subject := msg.Header.Get("Subject")
	from, _ := mail.ParseAddress(msg.Header.Get("From"))
	to, _ := mail.ParseAddressList(msg.Header.Get("To"))

	// Process the headers
	fmt.Println("Subject:", subject)
	fmt.Println("From:", from.Name, "<", from.Address, ">")

	fmt.Println("To:")
	for _, addr := range to {
		fmt.Println("-", addr.Name, "<", addr.Address, ">")
	}
}

In this example, we create a MIME message as a byte slice. We then use the mail.ReadMessage function to parse the message. The mail.ReadMessage function takes an io.Reader as its argument, so we use bytes.NewReader to create a reader from the byte slice.

After parsing the message, we use the Header.Get method to retrieve specific headers like "Subject", "From", and "To". mail.ParseAddress is then used to parse the "From" header into a mail.Address struct, while mail.ParseAddressList is used to parse the "To" header into a slice of mail.Address structs.

Finally, we can process the parsed headers and print them as desired.