Q: Recently upgraded to El Capitan I had this problem of Mail quitting unexpectedly when I viewed certain messages.  I followed the steps and was able to delete the problem messages.   Now I cannot restore the message viewer part of the mail window.

Hi there, alicefrompa!

Thanks for using the Apple Support Communities. From your post, I am understanding that you are not seeing the Message Viewer box that allows you to read your emails; you only see the inbox which includes a brief preview of the message, and are unable to regain access to it using the separator bars. I've had this happen to my own Mail app, and am happy to let you know how I was able to fix it!

You are actually on the right track--you do want to hover your mouse over the separator that goes between the inbox and Message Viewer sections (which should now be at the far right edge of your Mail window), and then drag it back to the left.  When you click over the inbox/Message Viewer, the cursor will turn into a solid black line with a single arrow pointing to the left. When you see the cursor change, click your left mouse button and (while holding the mouse button in) drag the line and arrow combination to the left; you may have to drag it a little farther than before to get it. I had to drag mine almost all the way to the middle of my Mail app before the Message Viewer box appeared. 

From there, you should be able to adjust the boxes to your desired widths as expected.

If you're still not able to get the Message Viewer visible after trying that, here are a few more things you can do.

1. Press Command + O on your keyboard.
    
2. Quit the Mail app, then press and hold the Shift key on your keyboard while reopening it.
    
3. Restart your Mac by selecting the Apple  menu in the upper right of your screen and choosing Restart from the drop down menu.

Cheers!

So glad to hear you were able to get your Message Viewer back!

Have a great day.