Use Mail.app from the Applications folder only


Mail.app should be located in the Applications folder, on the same volume as Mac OS X (you can choose Applications from the Go menu to see if it is there).
If you access Mail via an alias or Dock icon, Control-click or right-click the alias/icon and choose Show in Finder or Show Original from the contextual menu. The original will appear--it should be in the Applications folder (but not in an Applications folder that's inside a Previous Systems folder). If it is in Applications, go to the next section.
If Mail.app isn't in Applications, move it back to Applications. When you do, if you receive an alert that a newer version already exists in /Applications, do not replace it with the earlier version that you're dragging. Instead, delete the earlier version you've been using, and create a new alias or Dock icon from the newer Mail.app that is in /Applications. If the issue persists, go to the next section.

Remove third-party plugins
If the issue persists, remove any installed third-party Mail plugins that are installed:
- 

- Quit Mail if it's running. 

- From the Finder's Go menu choose Go to Folder. 

- Go to the ~/Library/Mail/Bundles folder. 

- Delete the contents of the folder. 

- From the Finder's Go menu choose Go to Folder. 

- Go to /Library/Mail/Bundles . 

- Delete the contents of the folder. 

- Open Mail. 


Install Mail Update 1.0 if necessary
Run Software Update. If Mail Update 1.0 appears, download and install it.
If Mail Update 1.0 does not appear, you do not need to install it.

Archive and Install if the issue persists
If none of the above steps resolve the issue, you may need to back up your data and perform an Archive and Install installation of Mac OS X 10.5 (select "Preserve Users and Network Settings" during the installation). After installation, update to Mac OS X v10.5.6 or later again.