Q: apple mail no longer starts with an open window

Quit Mail from the menu.

Restart Mail.

If that does not work:

Quit Mail from the menu.

Shutdown the mac, restart while hodling the Shift key, it is slow, login, then restart again normally.

Start Mail.

If you haven't already done so, restart the computer and see whether the problem goes away. If it doesn't, see below.

Please back up all data before proceeding.

Step 1

Open the Mail preferences window and select the Accounts tab. Select each of your mail accounts in turn and uncheck the box marked

          Enable this account

Then quit Mail. Force quit if necessary.

Step 2

If you're running OS X 10.11 ("El Capitan") or later, triple-click anywhere in the line below on this page to select it:

~/Library/Mail/V3/MailData

If you're running an older version of OS X, use this line:

~/Library/Mail/V2/MailData

Copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select

          Go ▹ Go to Folder

from the menu bar. Paste into the box that opens by pressing command-V, then press return.

A folder window will open. Inside it there should be files with names as follows:

          Envelope Index

          ExternalUpdates.storedata

Move those files to the Desktop, leaving the window open. Other files in the folder may have longer names that begin as above. Move those files, if any, to the Trash.

Step 3

Relaunch Mail. It should prompt you to re-import your messages. You may get a warning that the index is damaged and that Mail has to quit. Click OK. Typically, the process takes a few minutes, but it may take hours if you have gigantic mailboxes. In that case, you may be able to speed things up by temporarily adding your home folder to the Privacy list in the Spotlight preference pane. Remove it when Mail has finished importing.

If the import operation stalls or fails with an error message, quit Mail, delete the two new index files (keeping the original ones that you moved to the Desktop), and try again. According to many reports, the second attempt may succeed after the first one has failed.

Step 4

Reenable the accounts that you disabled in Step 1. Test. If Mail now works as expected, you can delete the files you moved to the Desktop. Otherwise, report your results.