This is about more than showing you will or can follow directions. For many agencies, a separate submissions inbox is a strategic workflow tool. By separating non-submission business from submissions, agents can quickly respond to things like editors’ offers over here, while they manage submissions over there. Often agents instruct authors of even expected submissions to use the general address, telling them to note in the subject line the referral or other exceptional status of the submission. So, submit the new project through the agency’s general submission address per posted guidelines. Within the query, write to the specific editor and lay out your history together. If someone else in the agency sorts the submissions, they’ll get it to the correct agent.