Shamim's Plugins

You can do any of the following.

1. You can set any email address you want in Front End PM PRO > Settings > Email > From Email. So when user receive an email, they will receive from this email. You can set an auto reply in that email to notify users that they will need to send message in your website

2. You can set Email Piping (https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/email-piping/) Or POP3 (https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/pop3-feature/) so that if user reply to any email, that will automatically send message in your website.

3. Disable email notification.

It should be one time change.
Do you have any other plugin which may have cron change functionality?

Refund processed.

You can easily customize if you want. Eg. to set subject as select field Use following code

add_filter( 'fep_form_fields', function( $fields ){
    if( isset( $fields['message_title'] ) ){
        $fields['message_title']['type'] = 'select';
        $fields['message_title']['options'] = [
            'Subject 1' => 'Subject title 1',
            'Subject 2' => 'Subject title 2',
        ];
    }
    return $fields;
});

This plugin uses WP core functions to operate. So it is not possible right now.

Please see https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/announcement-email-queue/

Please check following
1. Cron is working in your website.
2. Change email interval in Front End PM PRO > Settings > Emails and wait that time (important)

Please try following code

add_filter( 'fep_filter_rest_users_args', function( $args ){
    $args['search_columns'][] = 'user_email';
    return $args;
});

Please try this code

add_filter( 'fep_filter_user_name', function( $name, $id ){
    if( is_admin() && ! wp_doing_ajax() ){
        $name = fep_get_userdata( $id, 'user_email', 'id' );
    }
    return $name;
}, 10, 2 );

Are you suing any custom code to send message?