From the help it sounds like actions on one machine in the network, such as messages being marked seen, should immediately occur on other machines in my network when logged in as the same user. This is not happening.
The MailCOPA email client uses UDP port 7493 to talk to itself across your network. Make sure your firewall is configured to allow this port to operate on your network.
If for some reason this port is being used by other software you can change the port that the MailCOPA email program uses by entering the port number as a DWORD value at the registry entry pointed to by
HKEY_CURRENT_USER\InterVations\Software\MailCOPA\Options\Port
You will need to do this on every machine you have the MailCOPA email client installed on.
Tip: For an easier way of editing the registry consider using InterVations RegCOPA Registry Editor.