I can't add 127.0.0.1/8 to server > whitelist

[unsichtbarre]

I have recently had trouble sending messages with Horde, getting the message "There was an error sending your message: unable to add recipient [[email protected]]: Invalid response code received from server".
I checked the knowledgebase and it says to add 127.0.0.1/8 to the server whitelist, but plesk says "IP address/net mask pair invalid"

Are there any solutions? Help appreciated in advance.

-John