AppleMail preview pane missing

For El Capitan: see this Apple Support Communities discussion of the issue.

In AppleMail versions prior to El Capitan, I now see only the list of messages in my inbox. I no longer see the contents of the selected message appear in half of the window. How can I get back the preview of the message contents?

It's likely you either switched layouts due to an upgrade or accidentally swiped away the pane. How to fix it depends on which layout you prefer to use:

• The classic layout shows the preview pane below the message list.
• The default layout shows the preview pane to the right of the message list.

Classic Layout Solution

1. Be sure you are in classic view mode by going to Mail > Preferences > Viewing. Be sure the box next to "Use Classic Layout" is checked.
2. Click and drag up on the bottom edge of the Apple Mail window to view the message preview pane. You should see a little dot there. You can make it as large or small as you choose.

Default Layout Solution

1. Be sure you are not in classic view mode by going to Mail > Preferences > Viewing. Be sure the box next to "Use Classic Layout" is unchecked.
2. Click and drag the right edge of the Apple Mail window left to view the message preview pane. You should see a little dot there. You can make it as large or small as you choose.

If all else fails

There seem to be some cases where this is a corruption issue, of exactly what is unknown. We are following discussions in support and developer forums and will update as soon as more is known.

The following instructions are for advanced users only. It is possible to damage your installation of Mac OSX or lose data when following these instructions. Always do a backup of your system before attempting any potentially risky repairs or updates.

As of yet the only answer is to totally remove all your preference files related to Apple Mail and try again. More information is here: Apple Support Community's Corrupted Mail Preferences

If that still doesn't work, you may need to reinstall your operating system to get a clean version of Apple Mail. For more information, see Apple's Reinstall Mac OS X