Is there some sort of virus that blocks addresses in yahoo mail?

[xander]

Hi xTn,

If you did not make those changes to your blocked address list, it's possible that your account may have been compromised and the changes were done to prevent mail delivery. I highly advise that you change your password right away. That is the first and most important step in re-securing your account:

"Changing your password"
http://help.yahoo.com/kb/index?page=content&id=SLN2035

After you change your password be sure to follow the steps in this Yahoo Help article to finish re-securing your account by checking your account for any other changes that may have been made to the account and mail settings:

"Securing a hacked Yahoo account"
http://help.yahoo.com/kb/index?page=content&id=SLN3516

For tips you can use to keep your account safe I would highly recommend this Yahoo Help article:

"Safeguarding your Yahoo account"
http://help.yahoo.com/kb/index?page=content&id=SLN2080

Hope that helps!