"Email delivery failed" for members with custom domain emails

URL: https://blog.brainandbrain.co
Ghost: 4.1.0, on Digital Ocean

Since updating to 4.0, our latest post failed to send by email to certain members, all of which have custom domain emails (including my own, which had never failed to send before). Under “Member Activity,” I can see the error “Email delivery failed” for each. All the successfully sent ones appear to be common domains, like Gmail.

The only other details that may be worth mentioning are that the post itself was an image-heavy one and sent to paid members only.

I’ve checked the logs and don’t see any errors for that date, unless there’s somewhere else I should be looking for this specifically. Is this a known issue? Something on Mailgun’s end?

I’m seeing the same issue. No image-heavy posts, pure text.
I’d suspect it’s a Mailgun issue…