A few years ago a new email anti-spam fighting authentication tool DMARC was created. The system creates a pretty restricted way of authenticating email, the short of which says email from a domain is only legit if it comes certain ways. It’s a great way to prevent spoofing, but if implemented needlessly (it’s not recommended for everyone) it can easily break things.
It seems Yahoo implemented the system yesterday, and whether by accident or on purpose, there were some nasty side effects due to their upping security. Email not coming from the Yahoo outgoing email system that has a Yahoo domain is being rejected, not being delivered.
The short version: YOU CAN NO LONGER USE A YAHOO.COM ADDRESS FOR BULK EMAIL.
As a whole, it is not advised to send bulk email from “free” domains like Gmail, Yahoo, Hotmail, Outlook, Aol, etc. Many anti-spam rule sets have rules specifically that mark emails coming “From” addresses like those as a higher chance of being spam. Up to this point, you were rolling the dice to begin with as to how your email would be treated.
But, now with Yahoo’s new rule, not only is it not advisable, you can’t. And, there’s a chance we’ll see this with other domains as DMARC is adopted more.
If you don’t do so already, we suggest that all campaigns and organizations that send email from NGP and/or VAN use an organization address such as firstname.lastname@example.org. Not only do you avoid spam rules that might cause your email to not be seen, but you’ll avoid future changes by email service providers like the above.