Subthreshold depression is common, impairs functioning and increases the risk of major depression. Improving self-help coping strategies could help subthreshold depression and prevent major depression.To test the effectiveness of an automated email-based campaign promoting self-help behaviours.A randomised controlled trial was conducted through the website: www.moodmemos.com. Participants received automated emails twice weekly for 6 weeks containing advice about self-help strategies. Emails containing general information about depression served as a control. The principal outcome was depression symptom level on the nine-item Patient Health Questionnaire (PHQ-9) (trial registration: ACTRN12609000925246).The study recruited 1326 adults with subthreshold depression. There was a small significant difference in depression symptoms at post-intervention, favouring the active group (d = 0.17, 95% CI 0.01-0.34). There was a lower, although non-significant, risk of major depression in the active group (number needed to treat (NNT) 25, 95% CI 11 to ∞ to NNT(harm) 57).Emails promoting self-help strategies were beneficial. Internet delivery of self-help messages affords a low-cost, easily disseminated and highly automated approach for indicated prevention of depression.