Yahoo has recently made some email policy changes that will affect campaigns that you send using a Yahoo sender address. Yahoo, and other email systems utilizing DMARC, will not accept mail coming from a Yahoo email address that did not originate from Yahoo mail servers. You can read more about the Yahoo's changes here.
These changes Yahoo has made are in accordance with DMARC, or Domain-based Message Authentication, Reporting & Conformance. These changes have been made by Yahoo to reduce spam emails received from Yahoo addresses. You can learn more about DMARC here.
This change is very significant because any email system that utilizes DMARC will examine the sender email address to detect if a Yahoo sender email address originated from a Yahoo server. If it does not, messages will be filtered or potentially blocked entirely.
As a result of these changes, using a Yahoo address as your "Sender" or "From" address in SimplyCast is no longer allowed. If you are using a Yahoo address as your sender email address in SimplyCast you'll need to change it to something different by adding a new Sender Address.