Outpost is a communications layer that operates on top of your business email accounts. You may receive unwanted emails (also known as "spam") in your Outpost inbox from time to time. You can mark these messages as Spam within Outpost, but the controls for spam filters exist in your source email account.
Gmail has automated spam filters in place. If you need to add more specific blocks in your Gmail account, you can configure filters and blocked addresses using these instructions from Gmail.