How to resolve Gmail's SMTP error for unauthenticated senders on a new iMac Mail app?

I purchased a new Imac, and I cannot send email from my Mail app. They are returned with a message stating " SMTP error from remote mail server after end of data:

   550-5.7.26 Your email has been blocked because the sender is unauthenticated.

   550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.

   550-5.7.26

   550-5.7.26 Authentication results:

   550-5.7.26 DKIM = did not pass

   550-5.7.26 SPF [gmail.com] with ip: [84.***.35] = did not pass"

I spent 2 hours on the Apple support line, which concluded that what Gmail was asking me to do was only appropriate for a domaine owner, not just a Gamil account holder like me. It was a Google issue which only Google can resolve.

PS. I can still send gmails directly from the Gmail page, thus bypassing my Mail account. Also, when I try to change the SMTP account to Google, it reverts to blueyonder automatically!

Is anyone in the community able to help?

[Edited by Moderator]