Although Massive Open Online Courses (MOOCs) are increasing in popularity, they have been subject to criticism due to the high dropout rate. This study examined the impact of an instructor's personalized email intervention on the rate of completion of a nine week course, which included seven weekly quizzes, and the rate of completion of the final exam. The participants, who took an Israeli noncredit academic MOOC on negotiation management, were randomly assigned to two groups. Treatment group participants (N = 576) who did not complete the weekly quiz received a tailored reminder by email from their instructor encouraging them to complete the quiz and offering them assistance in order to deal with the past week's contents. The control group (N = 608) that did not complete the weekly quiz did not get any emails from the instructor. The impact of the intervention was measured in three different ways: the immediate-impact, the delayed-impact and a cumulative impact. The increase in quiz completion within a week after the instructor's email was defined as an immediate-impact. The increase in the completion of the next quiz was defined as a delayed impact. The increase in the final exam completion rates was defined as a cumulative impact. The results show that the weekly intervention had an immediate impact as well as a cumulative impact on the final exam completion rate. The results suggest that an instructor's acknowledgement and interest might increase learners' commitment to learning in a MOOC. This study aimed to gain insight into learners' propensity to stay active in a MOOC and to increase completion rates. Findings of this study can be useful to MOOC designers and instructors to design and facilitate more effective MOOCs for learners by using email interventions to prevent students from dropping out of courses.