src/MimeMailer/MimeMessage.php
protected function _buildInit()
    {
        $this->clear('message');
    
        $this->getMailer()
            ->setRegistry('boundary', $this->makeBoundary())
            ->setRegistry('boundary_ctt', $this->makeBoundary());
    
        // message type
        if (strlen($this->text)>0 && count($this->attachment)>0) {
            $this->getMailer()->setRegistry('message_type', 'multipart/mixed');
        } elseif (strlen($this->text)>0 && strlen($this->html)>0 && count($this->attachment)==0) {
            $this->getMailer()
                ->setRegistry('message_type', 'multipart/alternative')
                ->setRegistry('boundary_ctt', $this->getMailer()->getRegistry('boundary'));
        } else {
            $this->getMailer()->setRegistry('message_type', 'text/plain');
        }
    }
protected function _buildInit()
    {
        $this->clear('message');
    
        $this->getMailer()
            ->setRegistry('boundary', $this->makeBoundary())
            ->setRegistry('boundary_ctt', $this->makeBoundary());
    
        // message type
        if (strlen($this->text)>0 && count($this->attachment)>0) {
            $this->getMailer()->setRegistry('message_type', 'multipart/mixed');
        } elseif (strlen($this->text)>0 && strlen($this->html)>0 && count($this->attachment)==0) {
            $this->getMailer()
                ->setRegistry('message_type', 'multipart/alternative')
                ->setRegistry('boundary_ctt', $this->getMailer()->getRegistry('boundary'));
        } else {
            $this->getMailer()->setRegistry('message_type', 'text/plain');
        }
    }
Initialization before sending messages

@return void