Email delivery failed

One of my paid members is not receiving emails since I migrated from Substack to Ghost. I can see that they are not delivering; it says “email delivery failed” on his user page. He has a custom email address.

  1. Any idea why this might be? The user suggested that my website might be appearing on some blacklists.
  2. What can I do about it?
  3. Is there a way to check whether this is happening to anyone else? No one else has emailed me about it, but I have over 5,000 subscribers so I can’t go through and check them all manually.