Mail.app with El Capitan displays message titles in undersized width After I upgraded to OS X 10.11 (currently running 10.11.1), messages are no longer properly displayed in Mail.app: 

All message information is crammed into a space with minimum width, despite the fact that the actual column width is much larger.
Resizing the column temporarily restores a correct display but the problem comes back as soon as I switch to another mailbox. 

Of course restarting the application, or the computer itself, did not help.
I have not seen this problem documented elsewhere. Is it only me?
Delete the folder ~/Library/Containers/com.apple.mail

You may have to re-enter some preferences. I use MailTags and had to re-enter license data and preference information.