Yahoo email is very popular and user-friendly email service provider. But sometimes users have to face a very common error of "Yahoo mail error 18".Due to this customer not able to open their inbox. • We have listed below some reason for this error • If your OS is outdated you can get this error. • Due to virus attack. • If your browser is not updated this is also the reason behind this error. • Along with this if the cache is full in your system. But you can easily get rid of such issues by restarting your system, make sure that the browser is updated and cache should be cleared. If still the problem is not solved simply contact Yahoo Customer Service Number.