public static void startSMimeDecryption (final String encryptedPath, String decryptedPath) {
    logger.info("***  Start of sMIME decryption  ***"); //$NON-NLS-1$

    Properties props = System.getProperties();
    Session session = Session.getDefaultInstance(props, null);
    try {

        logger.info("Encrypted message: [" + encryptedPath + "]"); //$NON-NLS-1$ //$NON-NLS-2$
        File messageFile = new File(encryptedPath);
        String fileExtension = getFileExtension(messageFile);

        if (fileExtension != null) {

            MimeMessage m = null;

            if (fileExtension.toUpperCase().equals("EML")) { //$NON-NLS-1$

                try (FileInputStream streamIn = new FileInputStream(encryptedPath);) {
                    logger.info("Created FileInputStream."); //$NON-NLS-1$

                    m = new MimeMessage(session, streamIn);
                    logger.info("Created MimeMessage."); //$NON-NLS-1$
                } catch (IOException e) {
                    logger.error("Error while trying to read in message via stream. " + e.getMessage());
                    return;
                }   

            } else if (fileExtension.toUpperCase().equals("MSG")) { //$NON-NLS-1$
                m = createMimeMessageFromMsg(session, messageFile);
                logger.info("Created MimeMessage."); //$NON-NLS-1$
            } else {
                throw new InvalidFileException("Not a supported file type (extension). Only '*.eml' and '*.msg' are accepted."); //$NON-NLS-1$
            }

            if ((m.getFileName() != null) && (m.getFileName().toUpperCase().equals("SMIME.P7M") || Pattern.compile("^\\s*application\\/(x-)?pkcs7-mime.*$", Pattern.DOTALL).matcher(m.getContentType()).matches())) { //$NON-NLS-1$ //$NON-NLS-2$

                m = decryptMessage(m);

                logger.info("Saving decrypted message to file..."); //$NON-NLS-1$
                if (decryptedPath == null) {
                    String tempPath = messageFile.getAbsolutePath().substring(0, messageFile.getAbsolutePath().lastIndexOf(File.separator));
                    decryptedPath = tempPath + File.separator + FilenameUtils.removeExtension(messageFile.getName()) + "_decrypted.eml"; //$NON-NLS-1$
                }

                try (OutputStream str = Files.newOutputStream(Paths.get(decryptedPath))) {

                    m.writeTo(str);

                } catch (IOException e) {
                    logger.error("Failed to write to output stream. " + e.getMessage()); //$NON-NLS-1$
                    return;
                }

                logger.info("Decrypted message: [" + decryptedPath + "]"); //$NON-NLS-1$ //$NON-NLS-2$

            } else {
                throw new NotEncryptedMessageException("Not an encrypted message. Breaking up decryption."); //$NON-NLS-1$
            }

        } else {
            throw new InvalidFileException("No file extension found."); //$NON-NLS-1$
        }

    } catch (MessagingException | UnrecoverableKeyException | KeyStoreException | NoSuchAlgorithmException 
             | CMSException | InvalidFileException | InvalidMessageException e) {

        logger.error("Failed to read message. " + e.getMessage()); //$NON-NLS-1$
    } catch (NotEncryptedMessageException e) {
        logger.error(e.getMessage());
        System.exit(RETURNCODE_NODECRYPTION);
    }

    logger.info("***  End of sMIME decryption  ***"); //$NON-NLS-1$
}

private static MimeMessage createMimeMessageFromMsg (final Session session, final File messageFile) {
    MimeMessage mimeMsg = null;

    if (messageFile != null) {
        MAPIMessage mapiMsg = null;

        logger.info("Converting .msg to MimeMessage."); //$NON-NLS-1$
        try (NPOIFSFileSystem npoi = new NPOIFSFileSystem(messageFile)) {

            logger.info("Created NPOIFSFileSystem."); //$NON-NLS-1$

            mapiMsg = new MAPIMessage(npoi);
            logger.info("Created MAPIMessage."); //$NON-NLS-1$

            mimeMsg = new MimeMessage(session);
            logger.info("Creating MimeMessage..."); //$NON-NLS-1$

            // Header übertragen
            try {
                for (String current : mapiMsg.getHeaders()) {
                    try {

                        mimeMsg.addHeaderLine(current);

                    } catch (MessagingException e) {
                        logger.error("Could not add header line to MimeMessage. " + e.getMessage()); //$NON-NLS-1$
                    }
                }

                logger.info("Added header to MimeMessage."); //$NON-NLS-1$

            } catch (ChunkNotFoundException e) {
                logger.error("Could not retrieve header. " + e.getMessage()); //$NON-NLS-1$
            }

            // "Attachment" übertragen
            for (AttachmentChunks current : mapiMsg.getAttachmentFiles()) {
                try {

                    ByteArrayDataSource ds = new ByteArrayDataSource(current.getEmbeddedAttachmentObject(), "application/pkcs7-mime"); //$NON-NLS-1$
                    DataHandler dh = new DataHandler(ds);
                    mimeMsg.setDataHandler(dh);

                    logger.info("Added attachment to MimeMessage."); //$NON-NLS-1$

                } catch (MessagingException e) {
                    logger.error("Could not transfer attachment into MimeMessage. " + e.getMessage()); //$NON-NLS-1$
                }
            }

        } catch (IOException e) {
            logger.error("Error while trying to build MimeMessage. " + e.getMessage()); //$NON-NLS-1$
            mimeMsg = null;
        }
    }

    return mimeMsg;
}

private static MimeMessage decryptMessage(final MimeMessage encrypted) throws MessagingException, CMSException,
                                                                        KeyStoreException, UnrecoverableKeyException, 
                                                                        NoSuchAlgorithmException, InvalidMessageException {
    if (encrypted != null) {
        logger.info("Starting decrypting message..."); //$NON-NLS-1$
        KeyStore keystore = getKeyStore();

        SMIMEEnveloped message = new SMIMEEnveloped(encrypted);

        RecipientInformationStore recinfos = message.getRecipientInfos();
        Enumeration<String> aliases = keystore.aliases();
        RecipientInformation recid = null;
        String alias = null;

        logger.info("Decrypting message..."); //$NON-NLS-1$
        while ((recid == null) && aliases.hasMoreElements()) {
            alias = aliases.nextElement();
            if (keystore.isKeyEntry(alias)) {
                Certificate cert = keystore.getCertificate(alias);
                recid = recinfos.get(new JceKeyTransRecipientId((X509Certificate) cert));
            }
        }
        if (recid == null) {
            throw new RuntimeException("No decryption key found"); //$NON-NLS-1$
        }

        JceKeyTransEnvelopedRecipient recipient = new JceKeyTransEnvelopedRecipient((PrivateKey) keystore.getKey(alias, "changeit".toCharArray())); //$NON-NLS-1$

        byte[] content = recid.getContent(recipient);

        logger.info("Setting MimeMessage properties."); //$NON-NLS-1$
        MimeMessage decrypted = new MimeMessage(Session.getDefaultInstance(System.getProperties()), new ByteArrayInputStream(content));
        Enumeration<Header> headers = encrypted.getAllHeaders();
        while (headers.hasMoreElements()) {
            Header h = headers.nextElement();

            if (decrypted.getHeader(h.getName()) == null) {
                decrypted.addHeader(h.getName(), h.getValue());
            }

        }

        decrypted.saveChanges();            

        logger.info("Decrypted message."); //$NON-NLS-1$

        return decrypted;
    } else {
        throw new InvalidMessageException("Encrypted MimeMessage is null."); //$NON-NLS-1$
    }
}