Yahoo owns some technology called DomainKeys that can verify the sender of some emails. One thing it can do is recognise real and fake emails from eBay and PayPal. This is good because quite a few phishing emails claim to be from eBay or PayPal, intended to trick people into providing their login details.
Google has just implemented the technology for Gmail. So if safe email is of concern to you, your best bets are to use either Yahoo or Gmail for your emailing.
More technical information here.