If you are using an external DNS server, please use the following steps to change the external DNS setting on the SMTP virtual server:
1. Select Organization Configuration, Hub Transport Server, and Send connectors, respectively. From here, locate the clients send connector and right-click Properties.
Note: Use DNS MX records to route mail automatically.
2. If you want to route outbound mail, please use the DNS option to do so. Once the connector has used DNS to resolve the IP address of the remote SMTP server, then it will deliver the mail.
3. If you are going to use the DNS option, verify the following first:
Can your DNS server successfully resolve names on the Internet?
If you have used the Set-TransportServer cmdlet to configure an external DNS server that differs from the one specified on the network connection, can this external DNS server resolve names on the Internet?
4. Use the External DNS Lookup settings on the transport server to utilize the DNS servers list. Select this option if you want to use the external DNS servers list to resolve names on the Internet.
Note: Verify that you have configured the external DNS servers list by using the Set-TransportServer cmdlet or External DNS Lookup tab in the properties of the Hub Transport server object or Edge Transport server object.
5. From here, select the Use External DNS Lookup settings on the transport server check box.