Postfix - forward a single email address of a virtual domain to external server

I have an postfix server (mailserver) that provides mail services for a virtual domain (example.com). I have an email address in that virtual domain ([email protected]) that is used by an application running on an external server (appserver) that uses its own postfix installation to deliver mail. I would like all incoming mail for [email protected] to be forwarded to the appserver. I have made an entry in transport_maps as follows:

[email protected] smtp:appserver

But when I try send mail to the server I get "address does not exist in virtual mailbox table". How can I get the mail for [email protected] to be sent to the external server rather than be delivered locally?

thanks