I recently experienced an issue when sending e-mails from an Exchange 2003 mailbox to an Exchange 2010 mailbox during a 2003 to 2010 migration. Messages could be successfully sent from Exchange 2010 mailboxes to Exchange 2003 mailboxes but not the other way around, the messages would simply queue on the Exchange 2003 server. After a period of investigation it appeared this issue was occuring due to a smart host not being set against the SMTP connector in the Exchange System Manager on the Exchange 2003 server. To resolve the issue a smart host was configured on the SMTP connector via ESM to be the customers internet service providers upstream mail relay. The steps taken to resolve this issue are detailed below:
1. Connect to your Microsoft Exchange 2003 server and open the Exchange System Manager.
2. In the Exchange System Manager expand Servers -> Server Name -> Connectors.
3. Right click your SMTP connector and select properties.
4. On the general tab check the “Forward all mail through this connector to the following smart hosts” radio button and enter your internet service providers upstream mail relay, for example smtp.myisp.co.uk.
5. Test mail flow between the Exchange 2003 and Exchange 2010 environments.
That’s it, hopefully your migration mail flow issues will now be resolved.