Missing Apple Mail data after 10.5.6 upgrade?

A number of MacMedics’ clients and friends have been contacting us about an issue with Apple’s Mail after upgrading their Macs to 10.5.6. When they launch Mail after the update, there is no Mail viewer window available. This is kind of a frighting symptom because if you click on the “File” menu to open a “New Viewer Window” you would see a blank inbox and it would appear as if all messages and information was missing or lost.
Thankfully, it’s a pretty simple fix. But, before we go for the solution, this is a good time to remind folks that you should have a tested back up in place before you start moving files out of your Library folder in relation to a Mail or other system problems.
1. Go to Users > [Your User Name] > Library > Mail and move the files titled “MessageRules.plist” and “MessageRules.plist.backup” to the Desktop.
2. After moving the files to your Desktop, relaunch Mail.
3. After Mail relaunches Quit Mail again and move those two files back into their original location as mentioned in step 1.