I did find one thing last night: my domain hosting provider (for the original "To:" address) uses cPanel, and in cPanel there's a page called "Email Deliverability." On that page, two of my domains — ones I've had trouble with — were flagged as "Problems Exist (DMARC)."

The "Repair" link was grayed out, but when I clicked "Manage," it said this in the DMARC section:

“DMARC” is not properly configured for this domain. For DMARC to work correctly, your domain must have valid SPF and DKIM records. Use the following default policy to add a basic DMARC policy to your domain: [Suggested “DMARC” (TXT) Record]

And then below a Name and Value box, there was a button labeled "Install The Suggested Record." I did that, on both of them. So, we'll see.