This has been an useful thread for me. Since February 2022, I have had emails bouncing almost hourly that were sent to Yahoo.com, AOL.com and a couple of other email providers. I found that
unauthenticated email is the biggest culprit.
From the link: "Yahoo recently implemented new filtering algorithms and security measures. InboxAlly first detected the roll-out of changes starting in late February, 2022. Unexpected changes can wreak havoc for even the most pragmatic senders and require email marketers to adapt their sending practices in order to recover and ensure they can continue to reach subscribers’ inboxes." More details at the link.
Following the advice by
@nocte above, I found that we did not have
any SPF record installed on our email server or at our domain name service provider. I was getting a 6.2 /10 score returned by mail-tester.com.
After several unsuccessful configurations of a SPF TXT file I wasn't making any progress improving my score. So I coordinated with our server host who provided me with the correct SPF format for their email server authentication. I installed their suggested SPF DNS TXT record on our host server and on our domain name service provider (GoDaddy). Now I'm happy!!
Thank you everyone for your help on this.

I am getting the "you are not fully authenticated" note from mail-tester because "you do not have a DMARC record". Our shared server host does not support DMARC, which doesn't seem to make any difference. SPF is the important element.