I didn't see that post but, I tried what it says;
Changing the settings to Header's only, didn't help.
The person said;
Check if, from your yahoo account, you can adjust your forwarding options to only allow new mail. If this is an option mark your old messages as unread in portions that you feel Live Mail can handle without locking up. Synchronize your account with each new section.
Unfortunately, there is no such option in Yahoo!