I don’t think that’s a Discourse bug. The error message will occur when email is not properly configured. If you are in contact with the admins of any sites you are seeing this on, let them know about the problem. To fix it, they should make sure that all steps in the Setting Up Email section if the install guide have been completed: https://github.com/discourse/discourse/blob/main/docs/INSTALL-cloud.md#2-setting-up-email. My guess is that they haven’t added valid SPF and DKIM records to their domain.
You could also try pointing them to this topic: Email delivery configuration checklist.