Re-index Mail.
Go to your User's Library folder. (if you don't know how to find, see this link .)
• Scroll to Mail folder.
• Select the V2 > MailData folder.
• Delete any files in MailData with envelope in the name. (should be 3)
When you open Mail it will re-index your files.
If this fails, try this option to nuke all Mail settings.
The following steps are similar to testing in a new User.
1) Go to your User's Library folder. Do steps a,b,c
a) Scroll to Mail folder.
Rename Mail OLD. [When you open Mail it will create a new Mail folder. You can revert to this folder if needed by removing OLD from the name. You would need to rename the "new" Mail folder first.]
b) Go to Containers in the User's Library folder.
Delete the com.apple.mail folder.
c) Go to Preferences in the User's Library folder.
If you find any com.apple.mail.plist files, delete these.
2) Log out under the Apple in the Menu bar. Log in.
3) System Preferences > Internet Accounts > Delete all accounts.
4) Open Mail.
5) Enter one account at a time. Test to be sure you are connected before adding other accounts.
If you have Gmail, be sure you have the All Mail folder selected to sync. Workarounds we previously used no longer work in Mavericks. You might need to go into your Gmail Settings in your browser to make the necessary changes. See this article for help:
Use Gmail with Mavericks Mail
