You can use the MimeMessage API as Jeff mentioned, specially you want to use getContent() which will return the content as Java object and then you can check its it of type string or else its multipart. Below is the sample code you can try:

private String getContent(Part p) throws MessagingException,
            IOException {


        String finalContents = "";
        if (p.getContent() instanceof String) {
            finalContents = (String) p.getContent();
        } else {
            Multipart mp = (Multipart) p.getContent();
            if (mp.getCount() > 0) {
                Part bp = mp.getBodyPart(0);
                try {
                    finalContents = dumpPart(bp);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
        return finalContents.trim();
    }


private String dumpPart(Part p) throws Exception {

        InputStream is = p.getInputStream();
        if (!(is instanceof BufferedInputStream)) {
            is = new BufferedInputStream(is);
        }
        return getStringFromInputStream(is);
    }


private String getStringFromInputStream(InputStream is) {


        BufferedReader br = null;
        StringBuilder sb = new StringBuilder();
        String line;
        try {
            br = new BufferedReader(new InputStreamReader(is));
            while ((line = br.readLine()) != null) {
                sb.append(line);
                sb.append("\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (br != null) {
                try {
                    br.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return sb.toString();
    }

FYI: If you add DEBUG on "com.pega.pegarules.integration.engine.internal.services.email.EmailListener" class, you can see the MIME type used by incoming email in the PegaRULES log.