Hi Bourne,
Thanks for following up.
Before the migration, we did not test if @seconddomain.net can send / or receive external email, now because when a cloud-based user sends email, it goes to the internet before it is routed to the Exchange box. That is how we found the issue.
1. Exchange server 2011 SP3.
2. Yes we migrated to O365, the @seconddomain.net users are still in the on-premise Exchange 2003.
3. Both on-premise Exchange 2003 and Exchange 2011-based users can send and receive @seconddomain.net emails.
4. Yes the cloud-based users use @domain.onmicrosoft.com just for migration purpose. for daily use they are using @domain.com.
I think I should tell you more about our scenario.
With the seconddomain.net, we have had a website seconddomain.net which we registered at Melbourne IT that provides MX, SSL cert for the seconnddomain.net, etc. At the moment the MX record is pointing no where, that is why the NDR gave the error: Unrouteable address.
I get stuck on routing @seconddomain.net emails to / from external emails.
Regards