https://mailinator.com Great for testing site email, e.g. notifications. No need to set up any addresses, just send to firstname.lastname@example.org and then go to that inbox and your messages will show up there. Or not…
Using Dmarcian to manage domains spf, dkim, dmarc etc.
There are video tutorials at https://dmarcian.com/videos-on-all-things-dmarc/
Trying to avoid using 3rd party email services so as to maintain user privacy and generally not contribute to the control freaks efforts to track everything everyone does.
There is also https://www.mail-tester.com/spf-dkim-check
This is all part of a wider control freak operation that has been going on for a long time. It destroyed mailing lists and has made it much more difficult to run your own email service, which is what the people who created it would have wanted, of course.