Gmail and Yahoo are updating their mail requirements by extra checking with a feature called DMARC. DMARC, which stands for Domain-based Message Authentication, Reporting, and Conformance, -- is an email protocol that, -- when published for a domain, controls what happens if a message fails authentication tests -- the recipient server can`t verify that the message`s sender is who they say they are For full details review the MXtoolbox.com discussion on DMARK and how to resolve many issues. -- https://mxtoolbox.com/dmarc/details/what-is-dmarc