If you use a forwarding alias to direct mail to a POP mailbox, the spam filter setting of the the forwarding alias is used when mail arrives for that address, and the POP mailbox spam filter setting is ignored.
For example, if you create sales@example.com as a forwarding alias that forwards mail to a robert@example.com POP mailbox, and someone sends mail to sales@example.com, the setting that matters is the spam filter setting for sales@example.com.
The spam filter setting for robert@example.com would not make any difference for that particular message (but would be used if the message were sent directly to robert@example.com, of course).