Some macOS 15.5 Mail users are dealing with authentication issues when using the app with Microsoft or Google accounts. It's not yet clear why.

According to Slashdot, Google’s mail servers appear to be responsible for sending large amounts of backscatter. They don’t perform any recipient validation for the googlegroups and ...

Bulk Gmail and Yahoo Mail email senders will be required to strongly authenticate their emails following well-established best practices such as DMARC, SPF, and DKIM.