Q: Mail 9.0 opens with a new message window. Closing it causes mail to shut down unexpectedly. Help.

The mail unexpectedly closed comes with this code:

Process: Mail [918] Path: /Applications/Mail.app/Contents/MacOS/Mail Identifier: com.apple.mail Version: 9.0 (3094) Build Info: Mail-3094000000000000~1 Code Type: X86-64 (Native) Parent Process: ??? [1] Responsible: Mail [918] User ID: 501

I'm using El Capitan OS X 10.11.

My iMac is 21.5-inch, Late 2009