DirectAdmin uses two different kinds of email accounts. User email accounts, created automaticallly when the user is created, and virtual email accounts, which you create from your user login.
In the case of user email accounts when you create a forwarder, mail is no longer kept in the main mailbox.
In the case of virtual email accounts the mailbox and the forward are totally independent. If you have both a virtual email account and a forwarder, incoming email will both be forwarded and end up in the virtual mailbox.
Jeff