Has anyone heard of Certified Email?
I just saw this in my Yahoo! email this morning:
http://help.yahoo.com/l/us/yahoo/mail/y … xt-08.html
CertifiedEmail is a paid service provided by GoodMail that is generally targted at the big end of town. Provided you meed some strict criteria, you can almost guarantee inbox delivery to Yahoo (and other ISP's). At the very least, you need to display at least a year of good sending history and have been sending from a dedicated IP address (which we don't offer) for more than 6 months.
If your pockets are well lined (expect to pay a few cents per email) and you're experiencing deliverability problems with Yahoo, it might be worth considering. But, if you're not having any deliverability issues now, I genuinely don't think it's something worth paying for.