When the Outlook plugin is installed and the user attempt to File an email to the CRM.
The Outlook plugin is giving the error prompt: "Server CRM has an invalid respond"
The plugin is not able to connect with the CRM.
The plugin is using the wrong port
The register needs to be updated.
In regedit go to: HKEY_CURRENT_USER\Software\Sage\Exchange Outlook Plugin
Double click on the Port REG_DWORD make sure that the Port number is 80 in decimal.
Then do an iisreset and close and open outlook.
If it is anything other than 80 here it will result in the error that you mentioned.