wp-includes/PHPMailer/PHPMailer.php:2942Returns the whole MIME message.
string public function getSentMIMEMessage() { return static::stripTrailingWSP($this->MIMEHeader . $this->mailHeader) . static::$LE . static::$LE . $this->MIMEBody; }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.