Mac Mail Problem: Why are my mac.com emails being blocked by Google's Gmail? Error 550-5.7.26

Suddenly, all my emails from Apple's long-standing mac.com domain are going into other people's Gmail spam folders. Why?

I had a mail delivery failure message saying my email had been blocked because it was 'unauthenticated' and said it was a 'permanent error'.

It added: "SMTP error from remote server for TEXT command, host: gmail-smtp-in.l.google.com (74.125.206.26) reason: 550-5.7.26 This mail is unauthenticated, which poses a security risk to the 550-5.7.26 sender and Gmail users, and has been blocked. The sender must 550-5.7.26 authenticate with at least one of SPF or DKIM. For this message, 550-5.7.26 DKIM checks did not pass and SPF check for [mac.com] did not pass 550-5.7.26 with ip: [212.227.126.134]."