To rule out any directory problems on your boot volume first, run Disk First Aid repair on your boot volume via Disk Utility when booted from your Tiger install disc. Don't use Verify - select Repair Disk. If any problems are reported and successfully repaired, run Repair again until no problems are reported.
http://docs.info.apple.com/article.html?path=DiskUtility/10.5/en/duh1018.html
Next, quit Mail first and using the Finder go to Home > Library > Mail. Copy the Mail folder and place the copy on the Desktop for backup purposes.
Using the Finder, go to Home > Library > Mail > Envelope Index.
Delete the Envelope Index file and empty the Trash.
The Envelope Index file stores an index of all mailboxes and messages within.
Launch Mail and you will be prompted to import mailboxes. Select OK and allow this process to complete. This isn't really importing anything but replacing the deleted Envelope Index file and re-indexing all mailboxes and messages.
After confirming all mailboxes are available, you can delete the copy of the Mail folder placed on the Desktop.
