Many thanks to Kevin. I was getting rejects from Yahoo and I found his DKIM post from page 4 and it resolved the issue--emails are going through now!
But question for him, Ron, Theo, and any of you other admin types...
I checked with easydmarc and I have SPF set up, but I do *NOT* have DMARC set up. Do I need it? Note: I am not a bulk sender. This is my personal email account (I just happen to have bought my last name as my domain name).
The instructions for DMARC recommend an iterative test process and monitoring results as you go... The only person I know with a Yahoo email is someone I'd prefer not to use as a test subject unless absolutely necessary.