ISPs (Internet service providers) such as Yahoo! and AOL have recently implemented DMARC, a technical specification, the goal of which is to help reduce the potential for email-based abuse such as spamming, spoofing, and phishing. This means that if your email is being sent from a “Yahoo!” or “AOL” email address, or from any ISP that honors the DMARC policy (other ISPs are likely to follow suit), it may be rejected.
What can you do?
The best option is to set your “sent from” email address to an email address on your organization’s domain, not a Yahoo! or other free email address. You can find out more on the DMARC website.