Here the fast answer and solution from Argosoft Support:
It happens, because the server specified in the SmtpServerList.xml file in your WebMail\App_Data folder is not correct. Default value is localhost. If your mail server runs on a specific IP address (Options - Server Options - IP Binding on your mail server UI), or if you are accessing it remotely, on other computer, then that value has to be updated.
Go to the folder, where you have installed the web interface (by default, c:\Inetpub\wwwroot\WebMail), find the subfolder App_Data, open the file SmptServerList.xml, contained in that folder in the notepad, and change localhost between Host tags by the actual address of your server, it could ge an IP address (192.168.1.12) or domain name. If necessary, also update the Port value.
Thank you !!!