wp-includes/PHPMailer/PHPMailer.php:3343Get the boundaries that this message will use
array public function getBoundaries() { if (empty($this->boundary)) { $this->setBoundaries(); } return $this->boundary; }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/PHPMailer.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.