in src/main/java/org/apache/sling/commons/messaging/mail/internal/SimpleMessageBuilder.java [456:467]
private static void handleHtmlAndInlines(final MimeMultipart parent, final String html, final List<Inline> inlines) throws MessagingException {
if (!inlines.isEmpty()) { // html and inlines
final MimeMultipart related = new MimeMultipart(MULTIPART_SUBTYPE_RELATED);
addHtml(related, html);
addInlines(related, inlines);
final MimeBodyPart part = new MimeBodyPart();
part.setContent(related);
parent.addBodyPart(part);
} else { // html
addHtml(parent, html);
}
}