Three specific steps should be verified and/or performed to validate the Smart Host setup or if the Smart Host setup is not working:
1) If needed, change the remote created domain name to reflect the actual DNS-based domain name. Ex.: If the establishment is known as 'Example Company' with a domain name of 'www.examplecompany.com', use examplecompany.com for the remote created domain name.
2) Backup & remove any messages residing in the Badmail & Queue folders within the inetpub folder. These messages are found within <InstallDirectory>:\Inetpub\mailroot\Badmail & <InstallDirectory>:\Inetpub\mailroot\Queue
3) Enter the IP of the mail server in brackets (ie.: [188.8.131.52]) for the Smart Mail host field found in the following:
Internet Information Services > Select Default SMTP Virtual Server > Right Click or choose Action > Properties. Then select the Delivery Tab and choose Advanced. Under the Smart host field, enter the bracketed IP here and select OK. Then select Apply and choose OK.
These three steps should be followed to validate the Smart Host setup or if the Smart Host setup is not working.