PHPMailer\PHPMailer

PHPMailer::setMessageType │ protected │ WP 1.0

Set the message type. PHPMailer only supports some preset message types, not arbitrary MIME structures.

Method of the class: PHPMailer{}

No Hooks.

Returns

null. Nothing (null).

Usage

// protected - for code of main (parent) or child class
$result = $this->setMessageType();

PHPMailer::setMessageType() code WP 6.8.1

wp-includes/PHPMailer/PHPMailer.php 
protected function setMessageType()
{
    $type = [];
    if ($this->alternativeExists()) {
        $type[] = 'alt';
    }
    if ($this->inlineImageExists()) {
        $type[] = 'inline';
    }
    if ($this->attachmentExists()) {
        $type[] = 'attach';
    }
    $this->message_type = implode('_', $type);
    if ('' === $this->message_type) {
        //The 'plain' message_type refers to the message having a single body element, not that it is plain-text
        $this->message_type = 'plain';
    }
}