PHPMailer only sends email when SMTPDebug = true

Question

I'm using PHPmailer. It works when $mail->SMTPDebug = true; but when I remove that line, it silently fails. I say silently fails as it doesn't give any errors, and yet the email doesn't seem to be delivered.

        $mail = new PHPMailer;

        $mail->SMTPDebug = true;
        $mail->SMTPAuth = true;
        $mail->CharSet = 'utf-8';
        $mail->SMTPSecure = 'ssl';
        $mail->Host = 'smtp.gmail.com';
        $mail->Port = '465';
        $mail->Username = '[email protected]';
        $mail->Password = 'xxxxx';
        $mail->Mailer = 'smtp';
        $mail->AddReplyTo('[email protected]', 'xxxxx Support');
        $mail->From = '[email protected]';
        $mail->FromName = 'xxxxx Applications';
        $mail->Sender = 'xxxxx Applications';
        $mail->Priority = 3;

        //To us
        $mail->AddAddress('[email protected]', 'xxxxx xxxxx');
        //To Applicant
        $mail->AddAddress($email, $first_name.''.$last_name);
        $mail->IsHTML(true);

        $last_4_digits = substr($card_num, -4);
        //build email contents

        $mail->Subject = 'Application From '.$first_name.' '.$last_name;
        $mail->Body    = $body_html;
        $mail->AltBody = $body_text;

        if(!$mail->send()) {
           echo 'Message could not be sent.';
           echo 'Mailer Error: ' . $mail->ErrorInfo;
           exit;
        }

Answer 1

By setting

        $mail->SMTPDebug = false;

instead of omitting the line completely, it works every time.