Follow these steps:
Open the Go menu with the Option key pressed - Open Library - Containers - com.apple.mail - Data - Library - Preferences.
Locate the file com.apple.mail.plist and move it your desktop. Restart your mac and check whether everything works fine.
If this does not help, follow these steps:
- Shut down your Mac
- Wait until your computer turns off and after that press the Power button
- Right after you hear the startup tone, press and hold the Shift key
- Release the Shift key when you see a grey Apple sign and the progress bar below this sign
- Once you see Desktop, start a Disc Utility scan to detect and repair file system errors (don't forget to choose your main hard drive)
- Click on Verify Disc and then, if asked to fix problems, on Repair Disk
- After this, click on Verify Disc Permissions and then on Repair Disc Permissions
- After the process is finished, shut down your Mac and turn it back on after about 30 seconds
Hope this helps!