The google app engine documentation doesn't describe how to include an email header, how do you do it, i.e. How do you change this?
msg := &mail.Message{
Sender: "Example.com Support <support@example.com>",
To: []string{"email@bob.com"},
Subject: "Confirm your registration",
Body: fmt.Sprintf(confirmMessage, url),
}
if err := mail.Send(c, msg); err != nil {
c.Errorf("Couldn't send email: %v", err)
}
In the appengine/mail reference you can find that type Message has a field called Headers:
// Extra mail headers.
// See https://developers.google.com/appengine/docs/go/mail/overview
// for permissible headers.
Headers mail.Header
The type mail.Header can be found in the net/mail package, and only the following header names may be used, as described in the above overview link:
Example: (untested)
import netmail "net/mail" // mail is already taken by appengine/mail
...
msg := &mail.Message{
Sender: "Example.com Support <support@example.com>",
To: []string{"email@bob.com"},
Subject: "Confirm your registration",
Body: fmt.Sprintf(confirmMessage, url),
Headers: netmail.Header{"In-Reply-To": []string{"123456789"}},
}