Send WordPress Email Through the Postmark API

add_filter( 'pre_wp_mail', function ( $return, $atts ) {
    if ( null !== $return || ! defined( 'WPIDE_POSTMARK_TOKEN' ) || ! WPIDE_POSTMARK_TOKEN || ! empty( $atts['attachments'] ) ) {
        return $return;
    }

    $lines = is_array( $atts['headers'] ) ? $atts['headers'] : explode( "\n", str_replace( "\r\n", "\n", (string) $atts['headers'] ) );
    $h     = array( 'from' => '', 'reply-to' => array(), 'cc' => array(), 'bcc' => array(), 'content-type' => '' );
    foreach ( $lines as $line ) {
        if ( false === strpos( (string) $line, ':' ) ) {
            continue;
        }
        list( $name, $value ) = array_map( 'trim', explode( ':', $line, 2 ) );
        $name = strtolower( $name );
        if ( in_array( $name, array( 'reply-to', 'cc', 'bcc' ), true ) ) {
            $h[ $name ] = array_merge( $h[ $name ], array_filter( array_map( 'trim', explode( ',', $value ) ) ) );
        } elseif ( isset( $h[ $name ] ) ) {
            $h[ $name ] = $value;
        }
    }

    $from    = $h['from'] ? $h['from'] : sprintf( '%s <%s>', apply_filters( 'wp_mail_from_name', 'WordPress' ), apply_filters( 'wp_mail_from', get_option( 'admin_email' ) ) );
    $is_html = false !== stripos( $h['content-type'], 'text/html' ) || 'text/html' === apply_filters( 'wp_mail_content_type', 'text/plain' );
    $to      = is_array( $atts['to'] ) ? $atts['to'] : explode( ',', (string) $atts['to'] );

    $body = array(
        'From'          => $from,
        'To'            => implode( ',', array_map( 'trim', $to ) ),
        'Subject'       => (string) $atts['subject'],
        'MessageStream' => 'outbound',
    );
    $body[ $is_html ? 'HtmlBody' : 'TextBody' ] = (string) $atts['message'];
    if ( $h['cc'] ) {
        $body['Cc'] = implode( ',', $h['cc'] );
    }
    if ( $h['bcc'] ) {
        $body['Bcc'] = implode( ',', $h['bcc'] );
    }
    if ( $h['reply-to'] ) {
        $body['ReplyTo'] = implode( ',', $h['reply-to'] );
    }

    $response = wp_remote_post( 'https://api.postmarkapp.com/email', array(
        'timeout' => 15,
        'headers' => array( 'Accept' => 'application/json', 'Content-Type' => 'application/json', 'X-Postmark-Server-Token' => WPIDE_POSTMARK_TOKEN ),
        'body'    => wp_json_encode( $body ),
    ) );

    $code = (int) wp_remote_retrieve_response_code( $response );
    if ( 200 === $code ) {
        do_action( 'wp_mail_succeeded', $atts );
        return true;
    }

    $message = is_wp_error( $response ) ? $response->get_error_message() : 'Postmark returned HTTP ' . $code . ': ' . wp_remote_retrieve_body( $response );
    do_action( 'wp_mail_failed', new WP_Error( 'wp_mail_failed', $message, $atts ) );

    return false;
}, 10, 2 );