For the longest time, ISPs like Yahoo! and Hotmail have talked about how they’d be checking emails for authentication, then using little tiny icons to signify whether or not certain emails were "trustworthy."
I’ll admit, even though we’ve setup a few clients on DKIM and SenderID, we’ve yet to see any signs of those little icons. Sure, authentication has helped one of our MailChimp customers get through that mysterious "Bellsouth is blocking all HTML emails!" issue a while ago, but where’s that icon?
I just found it. I thought it would be in my inbox list pane (where it would be really helpful), but it actually shows up after you’ve already opened the email. Here it is for Andy Borowitz‘ email, when viewed in Yahoo (click on the thumbnail to the right to zoom in). I’d bet that when authentication gets more popular, ISPs will begin moving the little icon up into that inbox list, so you can use it to judge whether or not you should even open a message from somebody.
If you’d like to read more about how ISPs are using Authentication and Certification now, and how they plan to do it in the near future, here’s a recap of the recent Authentication and Online Trust Alliance.