For Exchange 2010; you may refer to below link
Below Screenshots are from Exchange 2013
- Open the EAC (Exchange Admin Center)
- From the left tab; click Mail Flow
- Click Send Connectors
- Click the + icon to create a New Send Connector
- Provide an appropriate name and select the type that is applicable to your work scenario
- Provide Authentication details if required for the connector
- Add the individual Fully Qualified Domain Names to the Address Space dialog box. But ensure the Cost field is marked as 1.
- The Send Connector with the least cost number will be used for sending mails to the domains specified. Please ensure the Cost value for any existing Send Connectors are changed from 1 to proceeding numbers.