MailWasher is a great companion to Yahoo as many people have found out, not only does Mailwasher allow you to preview all your Yahoo email first, it won't make the mistake of automatically adding your good email to a spam folder.
Adding your Yahoo email account to MailWasher is really easy - either is will import your email settings from your email program like Outlook or Thunderbird, or if will automatically setup itself in MailWasher when you add your email address and password. Then MailWasher will list all your emails in your Yahoo account, and you can read them all in MailWasher, mark any with a delete mark that you don't want, then press the 'Wash Mail' button to remove the unwanted emails so only the remaining good emails are left.
MailWasher lets you add unlimited numbers of Yahoo accounts, and also other email providers as well so you can view all your emails in one place.