Why the messages are not submitted?

As soon as you face any problems related to the plugin functionality, the first thing you need to do is check the Status tab and make sure all parameters meet the requirements. If any parameter is marked in red, follow a short instruction near the field or just contact your hosting support to enable/change the options

One more important parameter that can affect the plugin functionality but not yet listed in the status tab is the "fileinfo" extension, it must be enabled on your host in order to use mime_content_type function. To check this go to the PHP settings of your Cpanel and enable this option or contact your hosting support with this issue.