In this two cases what we should use? What or Which?
1) what/which email program do you use
2) what/which accounts have this problem
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
Generally "which" is used when there is a specific, identified set of choices. "What" is used when there is no such specified list. It's a little like the use of "the" versus "a".
So: "Everyone here must wear either a blue shirt or a red shirt. Which color do you want?" Versus, "You can wear any color shirt you like. What color do you want?"