Why is legitimate mail sent to my Yahoo! address not arriving?

I have tried sending mail to my Yahoo! Mail account and it has not arrived - this is the same information that a friend sent to my Yahoo! Mail account, basically a party invitation. Has anyone else had the same problem? I am uming it is being labelled as spam and rejected. I'm concerned that a whole range of legitimate emails are not getting through and this undermines my confidence in the Yahoo! Mail product.