A customer has been using SBS 2003 for years, and since last November we've been in the process of migrating them to newer platforms.
This customer has an office in the UK (their HQ) and an office in Australia. Up until a couple of years ago, they had separate .co.uk and .com.au domains, but now they have consolidated into a single .com domain
They use FuseMail (formerly EPA) for mail filtering, so all inbound emails hit the FuseMail servers first. In the FuseMail portal, it is possible to perform user level routing, so there is a group for the Australian users and any email addressed to them gets routed to the Exchange server in Australia, all other emails gets routed to the UK office. All good on the inbound.
The UK office currently has the original Exchange 2003 server as part of SBS as it's main mail server. All users (with the exception of a test account) use this as their mail server.
I've been following some documents for the migration of Exchange 2003 to Exchange 2010, and MOST of if it works ok. The problem I have affects user accounts on the Exchange 2010 server only.
If I send an email to an external user, or to a UK based internal user, then the emails get delivered ok, BUT if I send an email to an Australian user on the same .com domain, I get an immediate NDR from Exchange 2010 saying that the user is unknown.
What I did, was to change the Accepted Domain for the .com address in Exchange 2010 from Authoritative Domain to Internal Relay Domain. From here on in, I was able to successfully send out emails to external and Australian users, BUT
a whole heap of other problems then kicked in:-
Exchange 2003 started routing all internal emails out to FuseMail causing bounces etc., with no internal users getting emails from other internal users.
Inbound emails to UK users were being bounced because Exchange 2010 would not allow relaying
I need to be able to phase peoples mailboxes from Exchange 2003 to Exchange 2010 whilst retaining the ability / functionality of being able to send emails to users in Australia and without the problems above.
I've been able to reverse my above procedures, but even then, I had to stop and restart the Default SMTP Virtual Connector on Exchange 2003 in order for the queue to start clearing, but in with this, a lot of emails seem to have got lost in the ether somewhere.