How to encode and decode MIME parts with Content-Transfer-Encoding headers (e.g., 7bit, 8bit, base64) in Go?

To encode and decode MIME parts with various Content-Transfer-Encoding headers in Go, you can use the net/mail package along with the encoding/base64 package. Here's an example of how to do it:

Import the required packages:

import (
    "net/mail"
    "encoding/base64"
)

To encode a MIME part:

func EncodeMIMEPart(input []byte, encoding string) string {
    switch encoding {
    case "base64":
        return base64.StdEncoding.EncodeToString(input)
    case "quoted-printable":
        // Implement quoted-printable encoding logic
        // ...
    default:
        return string(input)
    }
}

To decode a MIME part:

func DecodeMIMEPart(input string, encoding string) ([]byte, error) {
    switch encoding {
    case "base64":
        return base64.StdEncoding.DecodeString(input)
    case "quoted-printable":
        // Implement quoted-printable decoding logic
        // ...
    default:
        return []byte(input), nil
    }
}

To encode a MIME message:

msg := &mail.Message{
    // Set other required MIME headers
    Header: mail.Header{
        "Content-Transfer-Encoding": []string{"base64"}, // Set the encoding header
    },
    Body: []byte("Hello, world!"), // Set the body of the message
}

encodedBody := EncodeMIMEPart(msg.Body, msg.Header.Get("Content-Transfer-Encoding"))
msg.Body = []byte(encodedBody)

To decode a MIME message:

msg, err := mail.ReadMessage(rawMessage) // Assume rawMessage is a byte array containing the entire message
if err != nil {
    // Handle error
}

encoding := msg.Header.Get("Content-Transfer-Encoding")
decodedBody, err := DecodeMIMEPart(string(msg.Body), encoding)
if err != nil {
    // Handle error
}

// Use the decoded body

Note: The example provided uses the base64 encoding type. If you need to support other encodings such as quoted-printable, you will have to implement the respective encoding and decoding logic inside the switch cases.

Remember to handle errors appropriately in your code.