We have a domain and my email address is my name at our domain. I send emails using Outlook and a SMTP server, whose domain is different from our domain. I believe that all of my DNS entries are set up correctly (related question below). Question: In the DMARC reports that I receive daily, I see DKIM Authentication Fail from Yahoo. Why?

Related question: When I checked on our domain from here, https://mxtoolbox.com/DNSLookup.aspx and then clicked on Find Problems, I was informed that there are issues with my SOA. I checked my DNS records and I see no SOA record. Should one be added? If so, how if the value determined?