Hello. For those having problem or trouble using PHPMailer Email Application and Roundcube Email Client when bouncing your email message, please use and add this code. You must know PHPMailer code in order to understand this code.
$PHPMailer->AltBody = ' '; or $PHPMailer->AltBody = ' Space String ';
Use $PHPMailer->AltBody = 'Blank Email Message' or string with one space;
Use this code before $PHPMailer->send();