This is a bit messy but i can be done like this:
1 Create user account with appropriate email address and set default password.
2 Create a contact with forwarding email address as “SMTP address”
3 Go to User account, Properties, Exchange General, delivery options, choose contact that was created for the final delivery address (the forward address)
4 Go to User account, Exchange advanced, Hide from address book.
5 Go to Contact properties, Exchange advanced, Hide from address book.
6 Disable user account. (It will never be used or logged into)