Email delivery problem

I'm having odd email delivery problems. Some of my emails are not getting delivered. And I believe I've narrowed down the cause, but have never heard of anything similar. DKIM, SPF, and DMARC are all set properly. If I send a simple email to [login to view URL], I get 10/10 score. HOWEVER, if I include my domain name in the email message BODY, it doesn't get delivered, not even to mail-tester.com. Something appears to be blocking my emails when they include only my domain name in the body (e.g., when replying to an email, or a new message with only my domain in the body msg). If the body of my email includes other domains like "[login to view URL]" or "[login to view URL]" then the emails get through. My domain is not blacklisted. But it appears that some spam blocker(s) marks email as spam if I just include my domain in the body. Does this seem possible? Is there any way to confirm, diagnose, and/or fix this? :(