private static byte[] encodeChar(int i) {
        char hex1 = CharUtil.toUppercaseHex((i >> 4) & 0xF);
        char hex2 = CharUtil.toUppercaseHex(i & 0xF);
        return new byte[]{ESCAPE_CHAR, (byte) hex1, (byte) hex2};
    }
https://www.rfc-editor.org/rfc/rfc5322#section-3.4
https://www.rfc-editor.org/rfc/rfc5322#section-3.6.2
turms-plugins/turms-plugin-push/src/main/java/im/turms/plugin/push/core/smtp/codec/MailEncoder.java