There is no configuration at Nabble to fix this. I understand that the message will appear only because the domain no longer exists. A user can still logon using an email address with a defunct domain, but obviously, no mail can be sent there.
Is the recipient an active member of the forum? If so, you could post explaining the problem and suggest they update their Personal Information
The forum administrator should have access to the true address and will be able to confirm whether the domain/email address works.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.