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