It appears that since last week, Yahoo has been rejecting emails with modified headers (such as forwarded emails) due to authentication check failures.

In one scenario, an email originally sent from Google’s @example.com to Google’s @example.net, and then auto-forwarded to @verizon.net, results in rejection by Yahoo.

In another scenario, when the forwarding is manually recreated following the same logic, the email ends up in the spam folder, particularly when a @yahoo.com account is the final destination in the forwarding chain.

This issue first surfaced around two weeks ago, although we have received only one report about it so far.

Has anyone else experienced this? Could there be an issue with how Yahoo interprets the ARC policy?