Email with one -reply to- address not sending

Using Apple Mail (13) MacOS and iPhoneOS. I put my personal custom domain email address into my "reply to" and use my Comcast.net IMAP server. My custom domain emails get redirected to my Comcast account.  Worked fine for years, until a few days ago, now these emails all show up fine in my SENT folder (including in the Comcast webmail interface) but they don't actually get sent. I've tested several modes and nothing shows up anywhere. No error messages or bounce notices. If I change the "reply to" to any other domain I use, it works fine. I receive all emails sent to my custom domain fine. Any other email account I have can send to my custom domain fine.

Is there a blacklist of some sort?