要提供两种选择，您可以将设置为
Content-Type，
mixed/alternative而不是
text/plain或
text/html：
如果我们想尝试了这一点，在API
Explorer中，我们只是使用base64编码的邮件，并使其安全网址，所有替换
/与
_和所有
+与
-。
在开发者控制台中：
btoa( "Subject: Example Subjectrn" + "From: <example@gmail.com>rn" + "To: <example@gmail.com>rn" + "Content-Type: multipart/alternative; boundary="foo_bar"rnrn" + "--foo_barrn" + "Content-Type: text/plain; charset=UTF-8rnrn" + "*Bold example message text*rnrn" + "--foo_barrn" + "Content-Type: text/html; charset=UTF-8rnrn" + "<div dir="ltr"><b>Bold example message text</b></div>rnrn" + "--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=="}要在Java中执行等效操作，您可以执行以下操作：
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.enprebase64URLSafeString(output.toByteArray());Message message = new Message();message.setRaw(rawEmail);message = service.users().messages().send(userId, message).execute();
