Sending email onward that was parsed using Golang net/mail.ReadMessage - email

Question

I'm looking for the cleanest way in Golang to transfer a message through (i.e. act as an SMTP proxy) while performing some manipulation on the message body html (e.g. adding an open tracking pixel - not yet coded).
The net/mail package includes a method ReadMessage that parses mail headers into a map, and gives you an io.Reader for the body. This is necessary to determine the MIME parts of the body for processing, rather than just io.Copying them through. (the simple stub version of this function, shown in the block comment, does just that).
The following function copies an incoming mail "src" to an outgoing mail stream "dest". (The calling code sets these up as DotReader and DotWriter which takes care of most of the "dot" processing needed for RFC5321.
// Processing of email body via IO stream functions
package main
import (
"bufio"
"io"
"log"
"net/mail"
"strings"
)
/* If you just want to pass through the entire mail headers and body, you can just use
the following alernative:
func MailCopy(dst io.Writer, src io.Reader) (int64, error) {
return io.Copy(dst, src)
}
*/
// MailCopy transfers the mail body from downstream (client) to upstream (server)
// The writer will be closed by the parent function, no need to close it here.
func MailCopy(dst io.Writer, src io.Reader) (int64, error) {
var totalWritten int64
const smtpCRLF = "\r\n"
message, err := mail.ReadMessage(bufio.NewReader(src))
if err != nil {
return totalWritten, err
}
// Pass through headers. The m.Header map does not preserve order, but that should not matter.
for hdrType, hdrList := range message.Header {
for _, hdrVal := range hdrList {
hdrLine := hdrType + ": " + hdrVal + smtpCRLF
log.Print("\t", hdrLine)
bytesWritten, err := dst.Write([]byte(hdrLine))
totalWritten += int64(bytesWritten)
if err != nil {
return totalWritten, err
}
}
}
// Blank line denotes end of headers
bytesWritten, err := io.Copy(dst, strings.NewReader(smtpCRLF))
totalWritten += int64(bytesWritten)
if err != nil {
return totalWritten, err
}
// Copy the body
bytesWritten, err = io.Copy(dst, message.Body)
totalWritten += int64(bytesWritten)
if err != nil {
return totalWritten, err
}
return totalWritten, err
}
It does seem necessary to build this, because there is no net/mail.WriteMessage() method.
the header order is always randomised by Golang's map functionality. This seems harmless in my tests
A forced CRLF needs to be put in between the end of the headers and the body, as per RFCs. DotWriter takes care of the terminating dot.
The function shown above works, I was wondering if there is a better way to do this?