In 2013, Gmail changed its interface and email categorization. Because of this change, emails sent by marketing platforms such as HubSpot will appear in the Promotions tab.
Your recipients can drag these email messages from the Promotions tab to Primary to help ensure that similar emails will appear there moving forward. Gmail will ask if you would like to do this for similar messages in the future; we recommend that you click Yes.
Please note: that the Promotions tab is by no means the equivalent of a spam folder. It is simply a newer email categorization interface Gmail implemented on its platform.