When user creates rule to forward all email to @isd.net account gwia sends error The message that you sent was undeliverable to the following:

user@isd.net (554 Unknown Sender (#5.1.8))

She can send message directly to user@isd.net
She can reply to user@isd.net

I tested this rule with hotmail account as well and it seems that any external email account that I enter in the rule gets the bounce back message.