An email that does not render on mobile phones would drive about 30 percent of recipients to click the unsubscribe button, according to a new report from BlueHornet. Mobile optimization is growing in importance. This number is up from 18 percent last year, indicating that now is the time to stop talking about mobile-friendly emails and start implementing them. The report also revealed that 24.5 percent of people unsubscribe because the email is no longer relevant.