Just create a rule in the database that does the forwarding. Be aware, that this might
routing loops, so exclude the forward recipient address from the forwarded mails.
If the database is not based on a mail template, then create the rule on the server or add the design elements needed for rules (forms rule and calendarprofile, folder $Rules, and needed libraries)
If you do not need any mail in the mailbox at all, then do what Panu mentioned in his comment: create a person document for the address (remove the mailin document) and enter a forwarding address there. Attention: you have to have unique addresses, so if you need the mails there, then don't do this...
Another possibility would be to assign the address that is used by the users to a group and change the "real" address of the mailin database to something else and add it as member of the group as well as the external recipient.
If you forward the mail outside of your network, then you most probably will have to configure any appliance between your server and the internet (e.g. a Clearswift, Ironport, etc.) to allow these mails, as for these devices the mail looks like coming from an external sender, sent to an external sender, but coming from an internal host, what makes it think this is a missrouted message or a relay attempt.
And you probably have to change the server configuration document of the server, that sends the smtp message to create a valid return path as shown here