Why the gmail API sends html emails as plain text?

Question

I'm trying to send a html email using the gmail API but for some reasons it randomly sends the email as plain/text. It seems that Google alters the content type header I set. Is there any reason for that? The email content is exactly same all the time (as I test it). Is the API still experimental? Sometimes when it works it also adds Content-Type: multipart/alternative; (although I never set it).

The encoding process looks as below. The code is Go but I guess it self explanatory and the process is language agnostic.

    header := make(map[string]string)
    header["From"] = em.From.String()
    header["To"] = em.To.String()
//   header["Subject"] = encodeRFC2047(em.Subject)
    header["Subject"] = em.Subject
    header["MIME-Version"] = "1.0"
    header["Content-Type"] = "text/html; charset=\"utf-8\""
//  header["Content-Transfer-Encoding"] = "base64"
    header["Content-Transfer-Encoding"] = "quoted-printable"
    var msg string
    for k, v := range header {
        msg += fmt.Sprintf("%s: %s\r\n", k, v)
    }

    msg += "\r\n" + em.Message
    gmsg := gmail.Message{
        Raw: encodeWeb64String([]byte(msg)),
    }
    _, err = gmailService.Users.Messages.Send("me", &gmsg).Do()

Question is similar to stackoverflow.com/questions/26841905/… — SGC

Eric D Eric D · Accepted Answer · 2015-01-14T17:54:00

Hmm, are you sure it's not a bug in your program? Can you print out the entire string and paste it here?

I just used the Gmail API to send an email like:

To: <redacted>
Subject: test html email 2015-01-14 09:45:40
Content-type: text/html

<html><body><b>hello</b>world</body></html>

and it looked as expected by the recipient's end in Gmail. Well, actually looks like it got wrapped it in a multipart/alternative and added a text/plain part as well (good thing IMO):

<random trace headers>
MIME-Version: 1.0
From: <redacted>
Date: Wed, 14 Jan 2015 09:46:41 -0800
Message-ID: 
Subject: test html email 2015-01-14 09:45:40
To: <redacted>
Content-Type: multipart/alternative; boundary=089e0141a9a2875c38050ca05201

--089e0141a9a2875c38050ca05201
Content-Type: text/plain; charset=UTF-8

*hello*world

--089e0141a9a2875c38050ca05201
Content-Type: text/html; charset=UTF-8

<html><body><b>hello</b>world</body></html>
--089e0141a9a2875c38050ca05201--

In any case, it's doing some parsing/sanitizing but does allow sending text/html email.

Why the gmail API sends html emails as plain text?

1 Answers