Yahoo began executing a stricter email policy that may unfortunately result in your newsletters landing up in the spam folder or being returned to you as bounced mail.
According to WordToTheWise Yahoo is doing this since, "there's been an ongoing attack against Yahoo users attempting to compromise their accounts and then send email from that Yahoo user to their contacts. Yahoo is telling other systems to not accept mail from Yahoo users if it doesn't come through Yahoo controlled servers. This includes the mail from the attackers, but also mail from regular Yahoo users."
If you're already sending through Emailbrain, you have a much higher success rate at having your emails delivered – we ensure optimal deliverability with feedback loops and we're a member of ReturnPath. However, if you send from a personal Yahoo address, your newsletter send will be made look fraudulent, even if it is not – due to the changes implemented by Yahoo.
These changes will affect receiving servers who respect DMARC policies, including Gmail, Hotmail, and others.
In order for you newsletter not be received in the spam folder or bounced back the best thing to do is send from an email address other than your personal Yahoo, if you have a different one. However, best practice is to set up a custom business domain for your sends. You may also create a free mail custom address on another email provider instead.