According to email security firm Cloudmark, 22% of messages sent through a Yahoo account are spam, compared to 11% from Microsoft and 6% from Google. Cloudmark attributes the difference to a recent history of layoffs and turnover at Yahoo, which makes retaining engineering talent difficult. Global spam volume has declined over the past few years, spiking at 6 trillion messages per month during 2008-2010 before settling to current steady levels of about 1 trillion messages per month.