The error is usually happening because of misconfiguration. If you use the sender email "sender@mydomain.com" and you have the "Track Responses" feature enabled at the same time. With the feature enabled, all of the emails that are purposed to be delivered to *@mydomain.com will be intercepted by LUCY, since LUCY expects a response to this domain. So an email to a recipient with *@mydomain.com domain will never be delivered.

To resolve the issue it is recommended to deactivate the "Track Responses" feature in all campaigns that have sender email matching with recipients domain or to use another sender email.