BACKGROUND: Sub-threshold depression is common, impairs functioning, and increases the risk of developing major depression. Although psychological treatments have been investigated for sub-threshold depression, they are costly. A less costly alternative could be an educational health promotion campaign about effective self-help for depression symptoms. The aim of the study is to test the efficacy of a low-cost email-based mental health promotion campaign in changing self-help behaviour and preventing more severe depression in adults with sub-threshold depression. METHODS/DESIGN: The project is a randomised controlled trial of an automated preventive email-intervention aimed at people with sub-threshold depression. Adults aged 18+ with sub-threshold depression (as measured with the Patient Health Questionnaire-9), who are not already receiving professional treatment for depression, are eligible for admission to the study. Internet users will sign up via the study website http://www.moodmemos.com and be randomly allocated to receive emails twice weekly for six weeks containing either self-help coping advice or general information about depression as a control. Outcomes will be assessed at the start, midpoint, and end of the intervention, as well as six months later. Outcomes assessed include symptoms, incidence of major depression, psychological distress, social and occupational functioning, coping strategies, and coping self-efficacy. The primary hypothesis is that the Mood Memo emails containing coping strategies will reduce depression symptoms and be better at preventing major depression than the control emails that contain general information about depression. DISCUSSION: Promotion of actions an individual can take to prevent physical disease is a technique often used in public health. This study applies this approach to mental health, and explores whether a low-cost, easily disseminated email-based campaign can improve self-help coping behaviour and prevent depression in adults with sub-threshold depression. TRIAL REGISTRATION: Australia and New Zealand Clinical Trials Register (ANZCTR): ACTRN12609000925246.