If you select Don't reopen, does it open?
IF YOU STILL HAVE PROBLEMS….
Try each step and check for results before trying next step.
1) Remove Mail saved state
• Quit Mail
• Go to User’s Library
• Go to the Containers > com.apple.mail > Data > Library > Saved Application State folder
• Locate and remove the folder called “com.apple.mail.savedState
• Re-launch Mail
2) Remove and rebuild Mail’s container
• Quit Mail
• Go to the User’s Library folder,
• Go to the Containers directory and move the folder called “com.apple.mail” to your Desktop
• Re-launch Mail
3) Repair Mail
• Quit Mail
• Go to the User’s Library folder,
• Go to Mail folder.
Select the V2 > MailData folder.
Delete any files in MailData with envelope in the name. (should be 3)
• Go to Preferences. If you find any com.apple.mail.plist files, delete these.
* Open Mail. Mail will reindex your files.
4) Test in a new User. If the problem goes away we know it’s an issue in your User’s folder.
• You’ll need your email passwords and setup available for the setup. You can save the text file in Shared folder for access in the new User.
• Go to System Preferences --> Create a New User in Users & Groups. Switch to the New User by logging out under the Apple in the Menu Bar.
• Add one email account at a time and let it sync up.
Let me know if this helps.
**Show Library Folder**
Select the Finder in the Dock. Under Go in the Menu bar > hold down the Option key and you’ll see the Library.