Send HTML email with plain text fallback via Gmail API

An answered question at StackOverflow suggests that adding html-markup to the body of the email will do the trick. Is that solution correct?

But what if the recipient's email server/client does not support HTML email?

E.g. when using Apache commons email, I do the following:

// set the html message
email.setHtmlMsg("<html>Our logo - <img src=\"cid:"+cid+"\"></html>");
// set the alternative message
email.setTextMsg("Your email client does not support HTML messages");

Is there any way to tell Gmail API what email to fall back to if the recipient email server/client does not support HTML?

P.S. I'm especially interested in Java code samples.

Thanks

Solution

To give two alternatives, you can set the Content-Type to mixed/alternative instead of either text/plain or text/html:

If we want to try this out in the API explorer, we just Base64-encode the mail, and make it url-safe by replacing all / with _ and all + with -.

In the Developer Console:

btoa(
  "Subject: Example Subject\r\n" +
  "From: <example@gmail.com>\r\n" +
  "To: <example@gmail.com>\r\n" +
  "Content-Type: multipart/alternative; boundary=\"foo_bar\"\r\n\r\n" +

  "--foo_bar\r\n" +
  "Content-Type: text/plain; charset=UTF-8\r\n\r\n" +

  "*Bold example message text*\r\n\r\n" +

  "--foo_bar\r\n" +
  "Content-Type: text/html; charset=UTF-8\r\n\r\n" +

  "<div dir=\"ltr\"><b>Bold example message text</b></div>\r\n\r\n" +

  "--foo_bar--" 
).replace(/\+/g, '-').replace(/\//g, '_');

POST https://www.googleapis.com/gmail/v1/users/me/messages/send?access_token={YOUR_API_KEY}

{
 "raw": "U3ViamVjdDogRXhhbXBsZSBTdWJqZWN0DQpGcm9tOiA8ZXhhbXBsZUBnbWFpbC5jb20-DQpUbzogPGV4YW1wbGVAZ21haWwuY29tPg0KQ29udGVudC1UeXBlOiBtdWx0aXBhcnQvYWx0ZXJuYXRpdmU7IGJvdW5kYXJ5PSJmb29fYmFyIg0KDQotLWZvb19iYXINCkNvbnRlbnQtVHlwZTogdGV4dC9wbGFpbjsgY2hhcnNldD1VVEYtOA0KDQoqQm9sZCBleGFtcGxlIG1lc3NhZ2UgdGV4dCoNCg0KLS1mb29fYmFyDQpDb250ZW50LVR5cGU6IHRleHQvaHRtbDsgY2hhcnNldD1VVEYtOA0KDQo8ZGl2IGRpcj0ibHRyIj48Yj5Cb2xkIGV4YW1wbGUgbWVzc2FnZSB0ZXh0PC9iPjwvZGl2Pg0KDQotLWZvb19iYXItLQ=="
}

To do the equivalent in Java, you could do:

Message message = new MimeMessage(session);
Multipart multiPart = new MimeMultipart("alternative");

MimeBodyPart textPart = new MimeBodyPart();
textPart.setText(text, "utf-8");

MimeBodyPart htmlPart = new MimeBodyPart();
htmlPart.setContent(html, "text/html; charset=utf-8");

multiPart.addBodyPart(textPart); 
multiPart.addBodyPart(htmlPart);
message.setContent(multiPart);

ByteArrayOutputStream output = new ByteArrayOutputStream();
message.writeTo(output);
String rawEmail = Base64.encodeBase64URLSafeString(output.toByteArray());

Message message = new Message();
message.setRaw(rawEmail);
message = service.users().messages().send(userId, message).execute();