isMail example

case 'reply_to':
                            $phpmailer->addReplyTo( $address, $recipient_name );
                            break;
                    }
                } catch ( PHPMailer\PHPMailer\Exception $e ) {
                    continue;
                }
            }
        }

        // Set to use PHP's mail().         $phpmailer->isMail();

        // Set Content-Type and charset.
        // If we don't have a Content-Type from the input headers.         if ( ! isset( $content_type ) ) {
            $content_type = 'text/plain';
        }

        /** * Filters the wp_mail() content type. * * @since 2.3.0 * * @param string $content_type Default wp_mail() content type. */