Each InboxSys received campaign has an ID. This ID is not visible in a message list. Here is how to find the real InboxSys message ID.
The number shown at the left column of each message list is unsuitable as identificator, because it is not static.
To see the message ID, open the message in question an click on the - symbol in the menu bar above the message.
You can now see the ID, right on top. To compile a direct link, take the base URL of the campaign (in most cases that is app.inboxsys.com/overview.php) and add the message ID as GET parameter. The name of the parameter is hid.