你需要包装你的 MimeMultipart 另一个 MimeBodyPart ，使用 MimeBodyPart.setContent（Multipart mp）方法。然后，您可以将 MimeBodyPart 添加到 mixedPart Object：

You need to wrap your MimeMultipart in another MimeBodyPart, using the MimeBodyPart.setContent(Multipart mp) method. Then you can add the MimeBodyPart to the mixedPart Object:

MimeMultipart alternativeMultipart = new MimeMultipart("alternative");

BodyPart textPart = new MimeBodyPart();
textPart.setContent("someText", "text/plain");
alternativeMultipart.addBodyPart(textPart);

BodyPart htmlPart = new MimeBodyPart();
htmlPart.setContent("someHtml", "text/html");
alternativeMultipart.addBodyPart(htmlPart);

MimeBodyPart alternativeBodyPart = new MimeBodyPart();
alternativeBodyPart.setContent(alternativeMultipart);

MimeMultipart mixedMultipart = new MimeMultipart("mixed");
mixedMultipart.addBodyPart(alternativeBodyPart);

MimeBodyPart textPart1 = new MimeBodyPart();
textPart1.setContent("someOtherText", "text/plain");
mixedMultipart.addBodyPart(textPart1);