To fix this problem is necessary to edit the HTTP proxy Header configuration of the WatchGuard firewall and add a new rule.
Open the HTTP proxy configuration, go to HTTP Response > Header Fields and add the rule:
Once the rule has been set, save the configuration to the device.
Accessing the Tiscali Webmail again, the problem is solved.
HTTP-proxy configuration sometime requires lot of time to be tuned but it’s the price for having a better security.