For a short time we hosted another entities email on our Exchange 2013 Server as part of a migration strategy. We have moved them to their own Exchange server. For the sake of explanation i'll call ours aus.com.au and theirs show.org.au
I removed (or so I thought) the config for the show.org.au, but when we attempt to send email to show.org.au our exchange server comes back with an NDR because it cant find the recipient in our server. Interestingly if the Recipient didnt exist on our server, the email is sent to the show.com.au's server, so it only effects those addresses that were hosted for a time on our server.
Clearly the show.org.au domain is still recorded somewhere in the Exchange 2013 config, but I havent been able to find it.
Get-AcceptedDomains - returns aus.com.au and aus.local
Get-ReceiveConnector configs show no clues