Hi there, & Welcome to Tom's Hardware!
With Windows 7, there is no normally installed email client. You have to choose one, then download or install it.
What kind of computer, Make, Model# do you have, which email program is giving you fits, and what email client have you used in the past?
If you downloaded one with another package, like Windows Live Essentials, or installed Outlook that comes with with several versions of Office, you can always uninstall it in the Control Panel, Programs & Features section, and choose your own. If you have more than 1 on your machinie, you will have to choose which one to make the default email client.