Back up all data.
Select all your mailboxes, and then select
Mailbox ▹ Export Mailbox...
from the Mail menu bar. Export the mailboxes to the Desktop folder.
Make a note of the settings for all your Mail accounts – everything you'd need to reconstruct the settings from scratch.
Quit Mail. Then select
▹ Force Quit...
from the menu bar. A small window will open with a list of running applications. Mail may appear in that list, even though you quit it. If so, select it and press return. Close the window.
In the Finder, hold down the option key and select
Go ▹ Library
from the menu bar. Move the following items from the folder that opens to the Trash (some may not exist):
- Caches/com.apple.mail
- Saved Application State/com.apple.mail.savedState
Leave the Finder window open for now.
Relaunch Mail and test. If the problem is solved, you’re done. Delete the exported mailboxes. Otherwise, quit Mail and move these items, if they exist, from the open Library folder to the Desktop:
- Application Support/AddressBook/MailRecents-v4.abcdmr
- Containers/com.apple.mail
- Mail
- Preferences/com.apple.mail.plist
- Preferences/com.apple.mail.searchhistory.plist
Note: you are not moving the Mail application. You’re moving a folder named “Mail.”
Relaunch Mail. It will behave as if you were setting it up for the first time. Go through the setup process with one of your accounts, using the information you noted earlier. Test. Same problem?
If there’s no improvement, quit Mail again and put back the items you moved to the Desktop, replacing any newer ones that may have been created in their place. You don’t need to replace the items you moved to the Trash.
If the issue is resolved, recreate the rest of your Mail settings, then import the mailboxes you exported:
File ▹ Import Mailbox...
After importing, test. If Mail is still working, delete the items on the Desktop and close the Finder window.