Thunderbird mail forwarding from a filter broken

I want to use a spare gmail account to backup all my incoming and outgoing mail in Thunderbird. Outgoing is easy enough - for each account under tools/accounts/Copies & Folders you add a line to the Bcc section. To set up backup of incoming mail, I created a filter to act on the inbox and find "read" messages, and forward them. This ought to work but doesn't....I just get an alert popup box with "sending of messages failed." It seems I'm not the only one.