thats if this attempt will propagate IMHO, Yahoo is being very irresponsible by implementing this new feature. An ESP's job, first, is to inter-operate with the existing Internet infrastructure. It will be a couple years at least before DMARC support is widely deployed enough in mailing lists to make this feasible. What happened to the spirit of cooperation on the Internet? Furthermore, this makes it difficult if not impossible to use a yahoo.comaddress with many 3rd-party mail clients. Yahoo is not only breaking email for their own customers, but for everyone else. For anyone interested, or who does not already know, there may be some work-arounds: http://dmarc.org/faq.html#s_3 Finally, a grassroots champagne should be started to move the average user away from Yahoo's mail system.