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 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=="
}
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.encodeBase64URLSafeString(output.toByteArray());
Message message = new Message();
message.setRaw(rawEmail);
message = service.users().messages().send(userId, message).execute();
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