I'm trying to email people on criagslist and when I click reply on there add my computer says cannot perform default mail clenit not set up proper how do I fix this?

Because, when you click the address, it automatically opens whatever email client came with your computer, usually outlook. If you haven't configured outlook, you can't use it. Just copy and paste the email address into the "To" portion of whatever email you use. Outlook also has a step by step configuration wizard if you want to set it up.

Why can't I send emails from craigslist