log4j-core/src/main/java/org/apache/logging/log4j/core/net/SmtpManager.java [132:249]:
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    @Override
    public void sendEvents(final Layout<?> layout, final LogEvent appendEvent) {
        if (message == null) {
            connect(appendEvent);
        }
        try {
            final LogEvent[] priorEvents = removeAllBufferedEvents();
            // LOG4J-310: log appendEvent even if priorEvents is empty

            final byte[] rawBytes = formatContentToBytes(priorEvents, appendEvent, layout);

            final String contentType = layout.getContentType();
            final String encoding = getEncoding(rawBytes, contentType);
            final byte[] encodedBytes = encodeContentToBytes(rawBytes, encoding);

            final InternetHeaders headers = getHeaders(contentType, encoding);
            final MimeMultipart mp = getMimeMultipart(encodedBytes, headers);

            final String subject = data.getSubjectSerializer().toSerializable(appendEvent);

            sendMultipartMessage(message, mp, subject);
        } catch (final MessagingException | IOException | RuntimeException e) {
            logError("Caught exception while sending e-mail notification.", e);
            throw new LoggingException("Error occurred while sending email", e);
        }
    }

    LogEvent[] removeAllBufferedEvents() {
        return buffer.removeAll();
    }

    protected byte[] formatContentToBytes(
            final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout) throws IOException {
        final ByteArrayOutputStream raw = new ByteArrayOutputStream();
        writeContent(priorEvents, appendEvent, layout, raw);
        return raw.toByteArray();
    }

    private void writeContent(
            final LogEvent[] priorEvents,
            final LogEvent appendEvent,
            final Layout<?> layout,
            final ByteArrayOutputStream out)
            throws IOException {
        writeHeader(layout, out);
        writeBuffer(priorEvents, appendEvent, layout, out);
        writeFooter(layout, out);
    }

    protected void writeHeader(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] header = layout.getHeader();
        if (header != null) {
            out.write(header);
        }
    }

    protected void writeBuffer(
            final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout, final OutputStream out)
            throws IOException {
        for (final LogEvent priorEvent : priorEvents) {
            final byte[] bytes = layout.toByteArray(priorEvent);
            out.write(bytes);
        }

        final byte[] bytes = layout.toByteArray(appendEvent);
        out.write(bytes);
    }

    protected void writeFooter(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] footer = layout.getFooter();
        if (footer != null) {
            out.write(footer);
        }
    }

    protected String getEncoding(final byte[] rawBytes, final String contentType) {
        final DataSource dataSource = new ByteArrayDataSource(rawBytes, contentType);
        return MimeUtility.getEncoding(dataSource);
    }

    protected byte[] encodeContentToBytes(final byte[] rawBytes, final String encoding)
            throws MessagingException, IOException {
        final ByteArrayOutputStream encoded = new ByteArrayOutputStream();
        encodeContent(rawBytes, encoding, encoded);
        return encoded.toByteArray();
    }

    protected void encodeContent(final byte[] bytes, final String encoding, final ByteArrayOutputStream out)
            throws MessagingException, IOException {
        try (final OutputStream encoder = MimeUtility.encode(out, encoding)) {
            encoder.write(bytes);
        }
    }

    protected InternetHeaders getHeaders(final String contentType, final String encoding) {
        final InternetHeaders headers = new InternetHeaders();
        headers.setHeader("Content-Type", contentType + "; charset=UTF-8");
        headers.setHeader("Content-Transfer-Encoding", encoding);
        return headers;
    }

    protected MimeMultipart getMimeMultipart(final byte[] encodedBytes, final InternetHeaders headers)
            throws MessagingException {
        final MimeMultipart mp = new MimeMultipart();
        final MimeBodyPart part = new MimeBodyPart(headers, encodedBytes);
        mp.addBodyPart(part);
        return mp;
    }

    /**
     * @deprecated Please use the {@link #sendMultipartMessage(MimeMessage, MimeMultipart, String)} method instead.
     */
    @Deprecated
    protected void sendMultipartMessage(final MimeMessage msg, final MimeMultipart mp) throws MessagingException {
        synchronized (msg) {
            msg.setContent(mp);
            msg.setSentDate(new Date());
            Transport.send(msg);
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log4j-jakarta-smtp/src/main/java/org/apache/logging/log4j/smtp/SmtpManager.java [88:205]:
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    @Override
    public void sendEvents(final Layout<?> layout, final LogEvent appendEvent) {
        if (message == null) {
            connect(appendEvent);
        }
        try {
            final LogEvent[] priorEvents = removeAllBufferedEvents();
            // LOG4J-310: log appendEvent even if priorEvents is empty

            final byte[] rawBytes = formatContentToBytes(priorEvents, appendEvent, layout);

            final String contentType = layout.getContentType();
            final String encoding = getEncoding(rawBytes, contentType);
            final byte[] encodedBytes = encodeContentToBytes(rawBytes, encoding);

            final InternetHeaders headers = getHeaders(contentType, encoding);
            final MimeMultipart mp = getMimeMultipart(encodedBytes, headers);

            final String subject = data.getSubjectSerializer().toSerializable(appendEvent);

            sendMultipartMessage(message, mp, subject);
        } catch (final MessagingException | IOException | RuntimeException e) {
            logError("Caught exception while sending e-mail notification.", e);
            throw new LoggingException("Error occurred while sending email", e);
        }
    }

    LogEvent[] removeAllBufferedEvents() {
        return buffer.removeAll();
    }

    protected byte[] formatContentToBytes(
            final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout) throws IOException {
        final ByteArrayOutputStream raw = new ByteArrayOutputStream();
        writeContent(priorEvents, appendEvent, layout, raw);
        return raw.toByteArray();
    }

    private void writeContent(
            final LogEvent[] priorEvents,
            final LogEvent appendEvent,
            final Layout<?> layout,
            final ByteArrayOutputStream out)
            throws IOException {
        writeHeader(layout, out);
        writeBuffer(priorEvents, appendEvent, layout, out);
        writeFooter(layout, out);
    }

    protected void writeHeader(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] header = layout.getHeader();
        if (header != null) {
            out.write(header);
        }
    }

    protected void writeBuffer(
            final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout, final OutputStream out)
            throws IOException {
        for (final LogEvent priorEvent : priorEvents) {
            final byte[] bytes = layout.toByteArray(priorEvent);
            out.write(bytes);
        }

        final byte[] bytes = layout.toByteArray(appendEvent);
        out.write(bytes);
    }

    protected void writeFooter(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] footer = layout.getFooter();
        if (footer != null) {
            out.write(footer);
        }
    }

    protected String getEncoding(final byte[] rawBytes, final String contentType) {
        final DataSource dataSource = new ByteArrayDataSource(rawBytes, contentType);
        return MimeUtility.getEncoding(dataSource);
    }

    protected byte[] encodeContentToBytes(final byte[] rawBytes, final String encoding)
            throws MessagingException, IOException {
        final ByteArrayOutputStream encoded = new ByteArrayOutputStream();
        encodeContent(rawBytes, encoding, encoded);
        return encoded.toByteArray();
    }

    protected void encodeContent(final byte[] bytes, final String encoding, final ByteArrayOutputStream out)
            throws MessagingException, IOException {
        try (final OutputStream encoder = MimeUtility.encode(out, encoding)) {
            encoder.write(bytes);
        }
    }

    protected InternetHeaders getHeaders(final String contentType, final String encoding) {
        final InternetHeaders headers = new InternetHeaders();
        headers.setHeader("Content-Type", contentType + "; charset=UTF-8");
        headers.setHeader("Content-Transfer-Encoding", encoding);
        return headers;
    }

    protected MimeMultipart getMimeMultipart(final byte[] encodedBytes, final InternetHeaders headers)
            throws MessagingException {
        final MimeMultipart mp = new MimeMultipart();
        final MimeBodyPart part = new MimeBodyPart(headers, encodedBytes);
        mp.addBodyPart(part);
        return mp;
    }

    /**
     * @deprecated Please use the {@link #sendMultipartMessage(MimeMessage, MimeMultipart, String)} method instead.
     */
    @Deprecated
    protected void sendMultipartMessage(final MimeMessage msg, final MimeMultipart mp) throws MessagingException {
        synchronized (msg) {
            msg.setContent(mp);
            msg.setSentDate(new Date());
            Transport.send(msg);
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