I can't quit Mail! It just keeps "clunking" when I try.

To Force Quit an application use

 > Force Quit ...

To force a power down hold down the power key until the power shuts off.

More on fixing Mail in the next post.

Others have reported the following solutions, depending on the situation.

Have an up-to-date Time Machine or other backup before attempting any major software updates or rebuilds.

For more about backups:

Time Machine Basics: http://support.apple.com/kb/ht1427

Most commonly used backup methods:

https://discussions.apple.com/docs/DOC-3045

Methodology to protect your data. Backups vs. Archives. Long-term data protection:

https://discussions.apple.com/docs/DOC-6031

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Make sure your software is up to date.

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If mail will not start crashes upon start see https://discussions.apple.com/thread/5624101?tstart=0

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Try booting in Safe Mode.

Boot and when you hear the bong hold down the Shift key.

It will boot slowly. Be patient.

Login the the user account.

Restart.

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A general repair to fix odd behaviors in Mail is Mailbox > Rebuild.

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Another fix:

Remove Container.plist from Library/com.apple.mail

In the Finder > Hold down the Option key and pull down to Library.

Open com.apple.mail.

Move Container.plist to the desktop and then delete it.

Start Mail.

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Another fix:

Shut down Mail.

Open the Finder. From the Finder menu bar click Go > Go to Folder

Copy and paste the following

~/Library/Mail/V2/MailData/

Click Go.

Move all "Envelope" files to your desktop.

Restart Mail and let it convert/reindex your mailboxes.

If this worked, Trash the Envelope files file that are on your desktop,

if not shut down Mail and put them back.

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Another fix:

Shut down Mail.

Open the Finder. From the Finder menu bar click Go > Go to Folder

Copy and paste the following:

~/Library/Containers/com.apple.mail/Data/Library/Caches/com.apple.mail/Cache.db

Click Go.

Move the Cache.db file to the Trash.

Restart your Mac and try Mail.

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Another fix:

Open the Finder. From the Finder menu bar click Go > Go to Folder

Copy and paste the following

~/Library/Mail/V2/MailData/

Click Go.

Move all the Accounts.plist to your desktop.

Restart Mail.

If this worked, Trash the Accounts.plist file that is on your desktop,

If not shut down Mail and put it back.

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You can try restoring mail from a Time Machine backup.

Hold down the Option key and in the Finder Go > Library

Enter Time Machine.

Select a date and time from when Mail was working.

Restore Mail.

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Advice from another thread: https://discussions.apple.com/thread/5466684?answerId=23657222022#23657222022