[imp] can you set a filter forward to external email?

[Robert Parker]

I select the Mail/Filters option from the tree on the left of the web
mail screen, then the Forward link. I enter my email address
(theparkers.mailbox(a)gmail.com) in the "Address(es) to forward to: "
box. Pressing the "Save" button shows the error:
There was an error activating the script. The driver said:
"Authentication Error"

Can horde only set a filter forward to accounts "within" horde or can
it filter forward to external email ? (in which case what have I done
incorrectly?)

Appologies in advance if this is a standard question, I did look
around the FAQ and searched the lists - couldn't find anything.

thanks
Robert