Shamim's Plugins

You can add your modified css in Dashboard > Front End PM PRO > Settings > General > Custom CSS

You are welcome.
Let me know anytime if you find any difficulty using this plugin.

You can add following code in your theme’s (child theme’s if any) functions.php

add_filter('fep_filter_message_before_send', function( $message ){
    $your_extra_user_ids = array( 1,2,3 );

    //Only for parent message
    if( empty($message['post_parent'] ) ){
        if( ! is_array( $message['message_to_id'] ) ){
            $message['message_to_id'] = array( $message['message_to_id'] );
        }
        $message['message_to_id'] = array_merge( $message['message_to_id'], $your_extra_user_ids );
    }
    return $message;
});

This shortcode does not support multiple recipient.
You can use fep_action_message_after_send hook to set participants to other user but may encounter some caching issue with message count. Instead You can use fep_filter_message_before_send and pass message_to_id as array of user ids. remember it already holds current recipient (may be as string), so add only other users.

To change recipients limit go to Dashboard > Front End PM PRO > Settings > Recipient
It works only with user ID.

Do you receive any error?
Please give me screenshot of
1. Dashboard > Front End PM PRO > Settings > Emails
2. Email which received in your email when you send a message in website
3. Any error when your reply to email

This plugin use wordpress restriction for file types. Please see https://codex.wordpress.org/Plugin_API/Filter_Reference/upload_mimes to add your file type.

Please follow https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/email-piping/ step by step.