Three states -- Colorado, Oregon, and Washington state -- have all-mail voting systems in place, but if recent history is any guide, they will soon be joined by other states like California, Arizona, Montana, Hawaii, Utah, and New Jersey.
The all-mail system is relatively new; Oregon was the first state to institute it after a referendum in 1998. Washington followed in 2011 and Colorado soon after in 2013. All three states introduced all-mail locally and its popularity led to its widespread use before it became the standard ballot system.