Q: El capitan reinstall without losing data?

Reinstall El Capitan Without Erasing the Drive

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.

Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the indented Macintosh HD entry from the the left side list.  Click on the First Aid button in the toolbar. Wait until the operation completes, then quit Disk Utility and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Continue button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless and more reliable.

Though following Kappy's excellent instructions will not delete your data, things can go wrong.  Therefore at all times you should have a backup (preferably two) which will protect you from the unexpected.

Ciao.

I have had a problem with Apple Mail for years. I have tried every fix posted in these forums. The best fix would be getting the app back to factory settings and starting over. I've been reading there is a way to do that, but I'd have to reinstall my OSX. Let's say I want to do that. Will Apple Mail really "start over" - meaning there will be no accounts or data associated with it? And what is up with the App store? Why would Apple not provide a means to download its own apps? Yes they are already on our operating systems, but why not give us a way to do a fresh install? If I am missing anything or if you can provide answers about re-installing El Capitan and how that might help me start fresh, I would really appreciate it!

Please, in the future you should post your own problem as a separate topic rather than hijacking an existing one from another user. To answer your question:

Install Yosemite or El Capitan from Scratch

 

     Backup if possible before continuing.

 

1. Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Reinstall OS X and click on the Continue button.

The question is what happens if I re-install El Capitan. If it wipes out data, that answers my question about what it will do to the Apple Mail app. So it is definitely a question about this topic. Sorry if I didn't make that more clear.

I think what you posted is how to go about re-installing. But the concern of the person who asked the question seems to be whether she will lose her data if she re-installs.

See "If you're reinstalling for other reasons" here.  As the article states, this does not remove your data:

How to reinstall OS X - Apple Support

Hopefully your Mac started in the state as indicated by "steps to prepare for selling or giving away your Mac" near the top.  If not, you should clear out the Mac and start again.

In case the Disk utility not going to work. Might be because of Invalid Extend entry issue. Give it a try,

Connect to a hard disk.

Install a new OS on it using cmd + r mode.

On restart use option mode(option key on boot chime) to boot Mac using external disk storage.

Copy the data you want to save in external disk.

Then format the data on Mac default disk and store fresh version.

Now you have your files on external hard disk and your ready to use Mac too.

Happy.