If you are receiving the following message within the Postbox preview area:
'www.postbox-inc could not be found'
'The browser could not find the host server address for the provided address'
It could be that Comodo Anti-Virus is blocking access through their Host Intrusion Protection System (HIPS), which works similar to a firewall with rulesets, a sandbox mode etc.
To fix this, make Postbox a trusted application in Comodo. Even though you allow Postbox to run via the firewall settings, HIPS settings can override the firewall settings.